KYIV, Ukraine — The presidents of four countries on Russia’s doorstep visited Ukraine on Wednesday and underscored their support for the embattled country, where they saw heavily damaged buildings and demanded accountability for what they called war crimes carried out by Russian forces.

The visit by the presidents of Poland, Lithuania, Latvia and Estonia was a strong show of solidarity by the leaders of the countries on NATO’s eastern flank, three of them — like Ukraine — once part of the Soviet Union.

The four traveled by train to the Ukrainian capital, Kyiv, to meet with their Ukrainian counterpart, Volodymyr Zelenskyy, and visited Borodyanka, one of the towns near Kyiv where evidence of atrocities was found after Russian troops withdrew to focus on the country’s east.

“The fight for Europe’s future is happening here,” Lithuanian President Gitanas Nauseda said, calling for tougher sanctions, including against Russia’s oil and gas shipments and all the country’s banks.

Elsewhere, in one of the most crucial battles of the war, Russia said more than 1,000 Ukrainian troops had surrendered in the besieged southern 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. But the ground advance slowly stalled and Russia lost potentially thousands of fighters in seven weeks of war. The conflict has killed untold numbers of Ukrainian civilians and forced millions more to flee. The fighting has also rattled the world economy, threatened global food supplies and shattered Europe’s post-Cold War balance.

A day after he called Russia’s actions in Ukraine “a genocide,” Biden approved $800 million in new military assistance to Ukraine, saying weapons from the West have sustained Ukraine’s fight so far and “we cannot rest now.” The weapons include artillery systems, armored personnel carriers and helicopters.

Appearing alongside Zelenskyy in an ornate room in Kyiv’s historical Mariinskyi Palace on Wednesday, the European leaders — , Estonian President Alar Karis, Polish President Andrzej Duda, Latvian President Egils Levits, and Lithuania’s Nauseda — reiterated their commitments to supporting Ukraine politically and with military aid.

“We know this history. We know what Russian occupation means. We know what Russian terrorism means,” Duda said. He said those who committed war crimes as well as those who gave the orders should be held accountable.

“If someone sends aircraft, if someone sends troops to shell residential districts, kill civilians, murder them, this is not war,” he said. “This is cruelty, this is banditry, this is terrorism.”

A 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 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.

Russian President Vladimir Putin has denied his troops committed atrocities, and on Tuesday insisted Russia “had no other choice” but to invade. He said 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.

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 that “the battle over the seaport is still ongoing today.”