KYIV, Ukraine (AP) — 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 — traveled 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. A Ukrainian official denied the claim, which 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. The war has also rattled the world economy, threatened 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 commended 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 a 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. Nauseda said the leaders visited Borodyanka, one of the towns near Kyiv where evidence of atrocities has been found.

“This is where the dark side of humankind has shown its face,” he wrote on Twitter. “Brutal war crimes committed by the Russian army will not stay unpunished.”

An expert report commissioned by the Organization for Security and Cooperation in Europe found “clear patterns of (international humanitarian law) violations by the Russian forces in their conduct of hostilities.” The report was written by experts selected by Ukraine and published Wednesday by the Vienna-based organization that promotes security and human rights.

The report said that there were also violations by Ukraine, but concluded those committed by Russia “are by far larger in scale and nature.”

Ukraine has previously acknowledged that there could be “isolated incidents” of violations and has said it would investigate.

Putin, however, 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 Moscow-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.”

Russian Defense Ministry spokesman Maj.-Gen. Igor Konashenkov said 1,026 troops from the Ukrainian 36th Marine Brigade had surrendered at a metals factory in the city. But Vadym Denysenko, adviser to Ukraine’s interior minister, rejected the claim, telling Current Time TV channel “the battle over the seaport is still ongoing today.”

It was unclear when the surrender may have 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.

Russian state television on Wednesday broadcast footage that it said was from the port city showing dozens of men in camouflage outfits walking with their hands up and carrying others on stretchers or in chair holds. One man held a white flag on a staff in one hand and the handle of a stretcher in another. In the background was a tall industrial building with its windows shattered and its roof missing, identified by the broadcaster as the Iliich metalworks.

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.”

Ukrainian Deputy Defense Minister Hanna Maliar said the country is investigating a claim that a drone dropped a poisonous substance on the city. She said it was possible phosphorus munitions had been used in Mariupol.

Phosphorus munitions are not formally classed as chemical weapons, but they cause horrendous burn, and deliberately firing them into an enclosed space could breach the Chemical Weapons Convention, said Marc-Michael Blum, a former laboratory head at the Organization for the Prohibition of Chemical Weapons.

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.

While Biden used the word “genocide” about Russia’s actions, he said it would be up to lawyers to decide if the country’s conduct met the international standard for genocide.

French President Emmanuel Macron declined to use the word but said “it has been established that war crimes have been committed by the Russian army.”

“We must find those responsible and bring them to justice,” he told France-2 television.

An International Criminal Court investigation into war crimes is underway in Ukraine, including into atrocities revealed after Moscow’s retreat from the Kyiv area, where Ukrainian authorities say more than 720 people were killed, with 403 bodies found in the town of Bucha alone.

ICC prosecutor Karim Khan, who visited Bucha, said in a tweet Wednesday that Ukraine “is a crime scene” and the court must “pierce the fog of war” to determine what has occurred.

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.”

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Stashevskyi reported from Yahidne, Ukraine. Associated Press writer Robert Burns in Washington, and AP journalists around the world contributed to this report.

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