With Russia claiming to have taken prisoner nearly 2,500 Ukrainian fighters from the besieged Mariupol steel plant, concerns grew about their fate as a Moscow-backed separatist leader vowed they would face tribunals. Russia has declared its full control of the Azovstal steel plant, which for weeks was the last holdout in Mariupol and a symbol of Ukrainian tenacity in the strategic port city, now in ruins with more than 20,000 residents feared dead. The seizure gives Russian President Vladimir Putin a badly wanted victory in the war he began nearly three months ago. As the West rallies behind Ukraine, Polish President Andrzej Duda arrived in Ukraine on an unannounced visit and will address the country's parliament on Sunday, his office said. Poland, which has welcomed millions of Ukrainian refugees since the start of the war, is a strong supporter of Ukraine's desire to join the European Union. With Russia blocking Ukraine's sea ports, Poland has become a major gateway for Western humanitarian aid and weapons going into Ukraine and has been helping Ukraine get its grain and other agricultural products to world markets. The Russian Defense Ministry released video of Ukrainian soldiers being detained after announcing that its forces had removed the last holdouts from the Mariupol plant’s extensive underground tunnels. It said a total of 2,439 had surrendered. Also read: Russia confirms halt of gas supply to Finland Family members of the fighters, who came from a variety of military and law enforcement units, have pleaded for them to be given rights as prisoners of war and eventually returned to Ukraine. Deputy Prime Minister Iryna Vereshchuk said Saturday that Ukraine “will fight for the return" of every one of them. Denis Pushilin, the pro-Kremlin head of an area of eastern Ukraine controlled by Moscow-backed separatists, said the captured fighters included some foreign nationals, though he did not provide details. He said they were sure to face a tribunal. Russian officials and state media have sought to characterize the fighters as neo-Nazis and criminals. “I believe that justice must be restored. There is a request for this from ordinary people, society, and, probably, the sane part of the world community,” Russian state news agency Tass quoted Pushilin as saying. Among the defenders were members of the Azov Regiment, whose far-right origins have been seized on by the Kremlin as part of its effort to cast the invasion as a battle against Nazi influence in Ukraine. A prominent member of Russia’s parliament, Leonid Slutsky, said Moscow was studying the possibility of exchanging the Azovstal fighters for Viktor Medvedchuk, a wealthy Ukrainian with close ties to Putin who faces criminal charges in Ukraine, the Russian news agency Interfax reported. Slutsky later walked back those remarks, saying he agreed with Pushilin that their fate should be decided by a tribunal. The Ukrainian government has not commented on Russia's claim of capturing Azovstal. Ukraine's military had told the fighters their mission was complete and they could come out. It described their extraction as an evacuation, not a mass surrender. Also read: In biggest victory yet, Russia claims to capture Mariupol The capture of Mariupol furthers Russia’s quest to create a land bridge from Russia stretching through the Donbas region to the Crimean Peninsula, which Moscow seized from Ukraine in 2014. The impact on the broader war remained unclear. Many Russian troops already had been redeployed from Mariupol to elsewhere in the conflict. Russian Defense Ministry spokesman Igor Konashenkov reported Saturday that Russia destroyed a Ukrainian special-operations base near Odesa, Ukraine's main Black Sea port, as well as a significant cache of Western-supplied weapons in northern Ukraine's Zhytomyr region. There was no confirmation from the Ukrainian side. The Ukrainian military reported heavy fighting in much of the Donbas in eastern Ukraine. “The situation in Donbas is extremely difficult," President Volodymyr Zelenskyy said in his nightly video address to the nation. “As in previous days, the Russian army is trying to attack Sloviansk and Sievierodonetsk.” He said Ukrainian forces are holding off the offensive “every day." Sievierodonetsk is the main city under Ukrainian control in the Luhansk region, which together with the Donetsk region makes up the Donbas. Gov. Serhii Haidai said the only functioning hospital in the city has just three doctors and supplies for 10 days. Sloviansk, in the Donetsk region, is critical to Russia’s objective of capturing all of eastern Ukraine and saw fierce fighting last month after Moscow’s troops backed off from Kyiv. Russian shelling on Saturday killed seven civilians and injured 10 more elsewhere in the region, the governor said. A monastery in the Donetsk region village of Bohorodichne was evacuated after being hit by a Russian airstrike, the regional police said Saturday. About 100 monks, nuns and children had been seeking safe shelter in the basement of the church and no one was hurt, the police said in a Facebook post, which included a video showing extensive damage to the monastery as well as nuns, monks and children boarding vans on Friday for the evacuation. Zelenskyy on Saturday emphasized that the Donbas remains part of Ukraine and his forces were fighting to liberate it. Speaking at a joint news conference with Portuguese Prime Minister Antonio Costa, he pressed Western countries for multiple-launch rocket systems, which he said “just stand still” in other countries yet are key to Ukraine's success. Portugal and Poland, where Costa stopped for talks before traveling on to Kyiv, support bringing Ukraine into the European Union quickly, even if some other EU members balk at granting it speedy access. U.S. President Joe Biden signed off Saturday on a fresh, $40 billion infusion of aid for Ukraine, with half for military assistance. Portugal pledged up to 250 million euros, as well as continued shipments of military equipment. Mariupol, which is part of the Donbas, was blockaded early in the war and became a frightening example to people elsewhere in the country of the hunger, terror and death they might face if the Russians surrounded their communities. The seaside steelworks, occupying some 11 square kilometers (4 square miles), were a battleground for weeks. Drawing Russian airstrikes, artillery and tank fire, the dwindling group of outgunned Ukrainian fighters held out with the help of airdrops that Zelenskyy said cost the lives of many “absolutely heroic” helicopter pilots. The Russian Defense Ministry on Saturday released video of Russian troops taking into custody Serhiy Volynskyy, the commander of the Ukrainian Navy's 36th Special Marine Brigade, which was one of the main forces defending the steel plant. The Associated Press has not been able to independently verify the date, location and conditions of the video. With Russia controlling the city, Ukrainian authorities are likely to face delays in documenting evidence of alleged Russian atrocities in Mariupol, including the bombings of a maternity hospital and a theater where hundreds of civilians had taken cover. Satellite images in April showed what appeared to be mass graves just outside Mariupol, where local officials accused Russia of concealing the slaughter by burying up to 9,000 civilians. An estimated 100,000 of the 450,000 people who resided in Mariupol before the war remain. Many, trapped by Russia’s siege, were left without food, water and electricity. The Ukrainian mayor of Mariupol warned Saturday the city is facing a health and sanitation “catastrophe” from mass burials in shallow pits across the ruined city as well as the breakdown of sewage systems. Vadim Boychenko said summer rains threaten to contaminate water sources as he pressed Russian forces to allow residents to safely leave the city. “In addition to the humanitarian catastrophe created by the (Russian) occupiers and collaborators, the city is on the verge of an outbreak of infectious diseases," he said on the messaging app Telegram.

Russia resumed pulverizing the Mariupol steel mill that has become the last stronghold of resistance in the bombed-out city, Ukrainian fighters said Monday, after a brief cease-fire over the weekend allowed the first evacuation of civilians from the plant. Meanwhile, a senior U.S. official warned that Russia is planning to annex large portions of eastern Ukraine this month and recognize the southern city of Kherson as an independent republic. Michael Carpenter, U.S. ambassador to the Organization for Security and Cooperation in Europe, said that those suspected actions are “straight out of the Kremlin’s playbook” and will not be recognized by the United States or its allies. In Mariupol, more than 100 people — including elderly women and mothers with small children — left the rubble-strewn Azovstal steelworks on Sunday and set out in buses and ambulances for the Ukrainian-controlled city of Zaporizhzhia, about 140 miles (230 kilometers) to the northwest, according to authorities and video released by the two sides. Mariupol Deputy Mayor Sergei Orlov told the BBC that the evacuees were making slow progress and would probably not arrive in Zaporizhzhia on Monday as hoped. Authorities gave no explanation for the delay. At least some of the civilians were apparently taken to a village controlled by Russia-backed separatists. The Russian military said some chose to stay in separatist areas, while dozens left for Ukrainian-held territory. In the past, Ukraine has accused Moscow’s troops of taking civilians against their will to Russia or Russian-controlled areas. The Kremlin has denied it. The Russian bombardment of the sprawling plant by air, tank and ship picked up again after the partial evacuation, Ukraine’s Azov Battalion, which is helping to defend the mill, said on the Telegram messaging app. Orlov said high-level negotiations were underway among Ukraine, Russia and international organizations on evacuating more people. Also Read: Evacuation of civilians from Ukrainian steel plant begins The steel-plant evacuation, if successful, would represent rare progress in easing the human cost of the almost 10-week war, which has caused particular suffering in Mariupol. Previous attempts to open safe corridors out of the southern port city and other places have broken down, with Ukrainian officials accusing Russian forces of shooting and shelling along agreed-on evacuation routes. Before the weekend evacuation, overseen by the United Nations and the Red Cross, about 1,000 civilians were believed to be in the plant along with an estimated 2,000 Ukrainian defenders. Russia has demanded that the fighters surrender; they have refused. As many as 100,000 people overall may still be in Mariupol, which had a prewar population of more than 400,000. Russian forces have pounded much of the city into rubble, trapping civilians with little food, water, heat or medicine. Some Mariupol residents got out of the city on their own, often in damaged private cars. As sunset approached, Mariupol resident Yaroslav Dmytryshyn rattled up to a reception center in Zaporizhzhia in a car with a back seat full of youngsters and two signs taped to the back window: “Children” and “Little ones.” “I can’t believe we survived,” he said, looking worn but in good spirits after two days on the road. “There is no Mariupol whatsoever,″ he said. “Someone needs to rebuild it, and it will take millions of tons of gold.” He said they lived just across the railroad tracks from the steel plant. “Ruined,” he said. “The factory is gone completely.” Anastasiia Dembytska, who took advantage of the cease-fire to leave with her daughter, nephew and dog, said she could see the steelworks from her window, when she dared to look out. “We could see the rockets flying” and clouds of smoke over the plant, she said. With most of Mariupol in ruins, a majority of the dozen Russian battalion tactical groups that had been around the city have moved north to other battlefronts in eastern Ukraine, according to a senior U.S. defense official who spoke on condition of anonymity to describe the Pentagon’s assessment. Ukrainian President Volodymyr Zelenskyy had said he hoped more people would be able to leave Mariupol in an organized evacuation on Monday. The city council told residents wanting to leave to gather at a shopping mall to wait for buses. Zelenskyy told Greek state television that remaining civilians in the steel plant were afraid to board buses because they feared they would be taken to Russia. He said he had been assured by the U.N. that they would be allowed to go to areas his government controls. Also Monday, Zelenskyy said that at least 220 Ukrainian children have been killed by the Russian army since the war began, and 1,570 educational institutions have been destroyed or damaged. In other developments, European Union energy ministers met Monday to discuss new sanctions against the Kremlin, which could include restrictions on Russian oil. Some Russia-dependent members of the 27-nation bloc, including Hungary and Slovakia, are wary of taking tough action. Thwarted in his bid to seize Kyiv, the capital, Russian President Vladimir Putin has shifted his focus to the Donbas, Ukraine’s eastern industrial heartland, where Moscow-backed separatists have been battling Ukrainian forces since 2014. Also Read: Evacuations underway in Mariupol; Pelosi visits Ukraine Carpenter, the U.S. ambassador to the OSCE, cited information that Russia is planning “sham referenda” in the so-called Donetsk and Luhansk “people’s republics” that would attach the entities to Russia. He also said there were signs that Russia would engineer an independence vote in Kherson. He noted that local mayors and legislators there have been abducted, that internet and cellphone service had been severed and that a Russian school curriculum is soon to be imposed. Ukraine’s government has said Russia also has introduced the ruble as currency there. Russia said Monday it struck dozens of military targets in the region in the past day. It said it hit concentrations of troops and weapons and an ammunition depot near Chervone in the Zaporizhzhia region, west of the Donbas. Ukrainian and Western officials say Moscow’s troops are raining fire indiscriminately, taking a heavy toll on civilians while making only slow progress. The governor of the Odesa region along the Black Sea Coast, Maksym Marchenko, said on Telegram that a Russian missile strike Monday on an Odesa infrastructure target caused deaths and injuries. He gave no details. Zelenskyy said the attack destroyed a dormitory and killed a 14-year-old boy. Ukraine said Russia also struck a strategic road and rail bridge west of Odesa. The bridge was heavily damaged in previous Russian strikes, and its destruction would cut a supply route for weapons and other cargo from neighboring Romania. The attack on Odessa came eight years to the day after deadly clashes between Ukrainian government supporters and protesters calling for autonomy in the country’s east. The government supporters in 2014 firebombed a trade union building containing pro-autonomy demonstrators, killing over 40 people. Also Monday, Ukraine claimed to have destroyed two small Russian patrol boats in the Black Sea. Mariupol, which lies in the Donbas, is key to Russia’s campaign in the east. Its capture would deprive Ukraine of a vital port, allow Russia to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014, and free up troops for fighting elsewhere. Britain’s Defense Ministry said it believes more than a quarter of all the fighting units Russia has deployed in Ukraine are now “combat ineffective” — unable to fight because of loss of troops or equipment.