Ukrainian President Volodymyr Zelenskyy marked the 500th day of the war Saturday by hailing the country's soldiers in a video from a Black Sea island that became the symbol of Ukraine’s resilience in the face of the Russian invasion.

Speaking from Snake Island, Zelenskyy honored the Ukrainian soldiers who fought for the island and all other defenders of the country, saying that reclaiming control of the island “is a great proof that Ukraine will regain every bit of its territory.”

“I want to thank — from here, from this place of victory — each of our soldiers for these 500 days,” Zelenskyy said. “Thank you to everyone who fights for Ukraine!”

It was unclear when the video was filmed. Zelenskyy was returning from Turkey on Saturday.

He announced that five commanders of the defense of the Azovstal steel plant, a grueling months-long siege early in the war, were returning on the plane with him.

The sprawling steelworks was the last bastion of resistance as Russian forces took control of the port city of Mariupol. Its defenders became renowned among Ukrainians for holding out in wretched conditions in the plant's tunnels and corridors.

Azovstal's more than 2,000 defenders left the steelworks in mid-May 2022 and were taken into Russian captivity. The five leaders, some of whom were part of the Azov national guard regiment that Russia denounces as neo-Nazi, were freed in a September prisoner swap and taken to Turkey.

Under the exchange, the leaders were to remain in Turkey until the end of the war under the Turkish president's protection. There was no immediate official explanation from Ankara or Kyiv about why they were allowed to return to Ukraine.

"The return of the leaders of the Azovites from Turkey to Ukraine is nothing more than a direct violation of the terms of the existing agreements. Moreover, in this case, the terms were violated by both the Ukrainian side and the Turkish side,” said Kremlin spokesman Dmitry Peskov.

Russian forces took control of Snake Island on Feb. 24, 2022, the day Moscow launched its invasion, in the apparent hope of using it as a staging ground for an assault on Odesa, Ukraine’s biggest port and the headquarters of its navy.

The island took on legendary significance for Ukraine’s resistance, when Ukrainian troops there reportedly received a demand from a Russian warship to surrender or be bombed. The answer supposedly came back, “Go (expletive) yourself.”

The island’s Ukrainian defenders were captured but later freed as part of a prisoner exchange. After the island was taken, the Ukrainian military heavily bombarded the small Russian garrison there, forcing the Russians to pull back on June 30, 2022. The Russian retreat reduced the threat of a seaborne Russian attack on Odesa and helped pave the way for a deal to resume Ukrainian grain exports.

Read: Ukraine and Russia accuse each other of preparing imminent attack on Europe's biggest nuclear plant

“Let the freedom that all our heroes of different times wanted for Ukraine and that must be won right now be a tribute to all those who gave their lives for Ukraine,” Zelenskyy said. “We will definitely win!”

Intense battles continued to rage Saturday in the country's east and south as Ukrainian forces pressed their attacks against multi-layered Russian defenses in the initial stages of their counteroffensive.

Ukraine's interior ministry said that a Russian rocket strike on the town of Lyman killed eight civilians and wounded 13 others early Saturday. Pavlo Kyrylenko, the governor of the eastern Donetsk region, posted images showing some of the dead, including a body lying under a bicycle and body fragments on the pavement next to a damaged vehicle, saying that “the Russian terrorists are continuing to strike civilians in Donetsk.”

Lyman is a few kilometers (miles) from the front line, where Russian troops have recently intensified fighting in the forests of Kreminna.

The U.K. Ministry of Defense said in its latest intelligence update that the eastern town of Bakhmut that was captured by the Russians in May has seen some of the most intense fighting along the front during the last week.

It said that Ukrainian forces have made steady gains to both the north and south of Bakhmut, noting that “Russian defenders are highly likely struggling with poor morale, a mix of disparate units and a limited ability to find and strike Ukrainian artillery.”

Amid the fighting, Russia and Ukraine accused each other of planning to sabotage the Russia-controlled Zaporizhzhia nuclear power plant, which is Europe's largest, fueling fears of a radiation catastrophe. Ukraine's military intelligence claimed Saturday that Russian troops have planted more mines around the plant, a claim that couldn't be independently verified.

Read:Ukraine accuses local man of directing missile strike that killed 10 at popular pizza restaurant

The head of the United Nations nuclear agency, Rafael Mariano Grossi, told The Associated Press on Friday that the International Atomic Energy Agency experts had recently gained access to more of the site, including the cooling pond and fuel storage areas, and found no mines there. Grossi said he was still pushing for access to the rooftops of reactors where Ukrainian officials accused Russia of planting explosives.

On Saturday, Russian Defense Minister Sergei Shoigu was shown visiting firing ranges where volunteer soldiers are being trained, a trip that comes two weeks after an abortive mutiny launched by mercenary chief Yevgeny Prigozhin, whose Wagner troops marched on Moscow in a bid to oust Shoigu.

Prigozhin agreed to end the mutiny, which represented the biggest threat to Russian President Vladimir Putin in his more than two decades in power, in exchange for an amnesty for himself and his troops and permission to move to Belarus. On Saturday, Russian messaging app channels ran comments by one of Wagner's commanders, Anton Yelizarov, who said that the mercenaries had taken leave but would eventually deploy to Belarus.

Pitched battles along the front line in Ukraine are raging as NATO leaders are set to meet in Vilnius for a two-day summit next week to offer more help in modernizing Ukraine’s armed forces, create a new high-level forum for consultations and reaffirm that it will join their alliance one day.

Ahead of the NATO summit, the U.S. has announced that it will provide Ukraine with cluster munitions, a move that President Joe Biden described as a “difficult decision.” Two-thirds of NATO members have banned the munitions, which have a track record of causing many civilian casualties, but the U.S. sees their delivery as a way to help bolster Ukraine's offensive and push through Russian front lines.

Ukrainian Defense Minister Oleksii Reznikov hailed the U.S. move, saying that the delivery of cluster munitions would help the country de-occupy its territories while saving the lives of the Ukrainian soldiers.

Read more: With Russia revolt over, mercenaries' future and direction of Ukraine war remain uncertain

Reznikov vowed that Ukraine would use the munitions only for the de-occupation of its territory and would not fire them at Russia's proper territory. He also noted that the Ukrainian military would not use cluster munitions in urban areas to avoid hurting civilians.