The presidents of four countries on Russia's doorstep visited Ukraine on Wednesday in a show of support for the embattled country, after Russian President Vladimir Putin vowed to continue his bloody offensive until its “full completion.”

The presidents of Poland, Lithuania, Latvia and Estonia — all NATO countries that worry they may face Russian attack in the future if Ukraine falls — were traveling by train to Kyiv to meet Ukrainian leader Volodymyr Zelenskyy.

In one of the most crucial battles of the war, Russia said more than 1,000 Ukrainian troops had surrendered in the besieged port of Mariupol, where Ukrainian forces have been holding out in pockets of the city. The information could not be verified.

Russia invaded on Feb. 24 with the goal, according to Western officials, of taking Kyiv, toppling the government and installing a Moscow-friendly one. In the seven weeks since, the ground advance stalled, Russia has lost potentially thousands of fighters — and the war has forced millions of Ukrainians to flee, rattled the world economy, threated global food supplies and shattered Europe's post-Cold War balance.

U.S. President Joe Biden on Tuesday called Russia's actions in Ukraine “a genocide” for the first time, saying “Putin is just trying to wipe out the idea of even being a Ukrainian.”

Zelenskyy applauded Biden's use of the word, saying “calling things by their names is essential to stand up to evil.”

"We are grateful for US assistance provided so far and we urgently need more heavy weapons to prevent further Russian atrocities,” he added in his tweet.

The European leaders visiting Ukraine planned to deliver “a strong message of political support and military assistance." Lithuanian President Gitanas Nauseda said.

Nauseda, Estonian President Alar Karis, Poland’s Andrzej Duda and Egils Levits of Latvia also planned to discuss investigations into alleged Russian war crimes, including the massacre of civilians.

Putin has denied his troops committed atrocities, and on Tuesday insisted Russia “had no other choice” but to invade, saying the offensive aimed to protect people in parts of eastern Ukraine and to “ensure Russia’s own security.” He vowed it would “continue until its full completion and the fulfillment of the tasks that have been set.”

He insisted Russia’s campaign was going as planned despite a major withdrawal after its forces failed to take the capital and suffered significant losses.

Following those setbacks, Russian troops are now gearing up for a major offensive in the eastern Donbas region, where Russian-allied separatists and Ukrainian forces have been fighting since 2014, and where Russia has recognized the separatists’ claims of independence. Military strategists say Moscow believes local support, logistics and the terrain in the region favor its larger, better-armed military, potentially allowing Russia to finally turn the tide in its favor.

Britain’s defense ministry said Wednesday that “an inability to cohere and coordinate military activity has hampered Russia’s invasion to date.” Western officials say Russia recently appointed a new top general for the war, Alexander Dvornikov, to try to get a grip on its campaign.

A key piece to that campaign is Mariupol, which lies in the Donbas and which the Russians have besieged and pummeled since nearly the start of the war. Ukrainian presidential adviser Mykhailo Podoliak tweeted that the city’s defenders were short of supplies but were “fighting under the bombs for each meter of the city.”

READ: More than 10,000 civilians dead in Ukraine port city: Mayor

Russian Defense Ministry spokesman Maj.-Gen. Igor Konashenkov said 1,026 troops from the Ukrainian 36th Marine Brigade had surrendered in the city. It was unclear when this occurred or how many forces were still defending Mariupol.

According to the BBC, Aiden Aslin, a British man fighting in the Ukrainian military in Mariupol, called his mother and a friend to say he and his comrades were out of food, ammunition and other supplies and would surrender.

Another Zelenskyy adviser Oleksiy Arestovych did not comment on the surrender claim, but said in a post on Twitter that elements of the same brigade managed to link up with other Ukrainian forces in the city as a result of a “risky maneuver.”

Ukraine is investigating a claim that a drone dropped a poisonous substance on the city. The assertion by the Azov Regiment, a far-right group now part of the Ukrainian military, could not be independently verified. The regiment indicated there were no serious injuries.

Ukrainian Deputy Defense Minister Hanna Maliar said it was possible phosphorus munitions — which cause horrendous burns but are not classed as chemical weapons — had been used in Mariupol, which has been pummeled by weeks of Russian assaults.

Deliberately firing phosphorus munitions into an enclosed space to expose people to fumes could breach the Chemical Weapons Convention, said Marc-Michael Blum, a former laboratory head at the Netherlands-based Organization for the Prohibition of Chemical Weapons.

Western officials warned that any use of chemical weapons by Russia would be a serious escalation of the already devastating war. Zelenskyy said that while experts try to determine what the substance might be, “The world must react now.”

In Washington, a senior U.S. defense official said the Biden administration was preparing another package of military aid for Ukraine to be announced in the coming days, possibly totaling $750 million. The official spoke on condition of anonymity to discuss plans not yet publicly announced.

Biden used the word “genocide” about Russia's actions during a visit to Iowa. He said it would be up to lawyers to decide if Russia’s conduct met the international standard for genocide, but said “it sure seems that way to me.”

Neither he nor his administration announced new consequences for Russia or assistance to Ukraine following the assessment.

An investigation into war crimes is already underway in Ukraine, including into atrocities revealed after Moscow’s retreat from cities and towns around Kyiv.

Zelenskyy said evidence of “inhuman cruelty” toward women and children in Bucha and other suburbs of Kyiv continued to surface, including alleged rapes.

More than 720 people were killed in Kyiv suburbs that had been occupied by Russian troops and over 200 were considered missing, the Interior Ministry said early Wednesday.

In Bucha alone, Mayor Anatoliy Fedoruk said 403 bodies had been found and the toll could rise as minesweepers comb the area.

Residents in Yahidne, a village near the northern city of Chernihiv, said Russian troops forced them to stay for almost a month in the basement of a school, only allowing them outside to go to the toilet, cook on open fires — and bury those who died in a mass grave.

In one of the rooms, the residents wrote the names of those who perished during the ordeal. The list counted 18 people.

“An old man died near me and then his wife died next,” said resident Valentyna Saroyan. “Then a man died who was lying there, then a woman sitting next to me. ... She died as well. Another old man looked so healthy, he was doing exercises, but then he was sitting and fell. That was it.”

Ukraine’s Deputy Prime Minister Iryna Vereshchuk said humanitarian corridors used to get people out of cities under Russian attack will not operate on Wednesday because of poor security.

Meanwhile, Ukrainian officials detained fugitive Ukrainian oligarch Viktor Medvedchuk, who is both the former leader of a pro-Russian opposition party and a close associate of Putin. Medvedchuk was under house arrest before the war began and disappeared shortly after hostilities broke out.