KYIV, Ukraine — Ukraine said its forces struck and seriously damaged the flagship of Russia’s Black Sea fleet, dealing a potentially major setback to Moscow’s troops as they try to regroup for a renewed offensive in eastern Ukraine after retreating from much of the north, including the capital.

Russia said today the entire crew of the Moskva, a warship that would typically have 500 sailors on board, was forced to evacuate after a fire overnight and also reported it was badly damaged. But it did not acknowledge any attack, which, in addition to any practical impact, would also deal a major blow to Russian prestige seven weeks into a war that is already widely seen as a historic blunder.

The damage to the ship came hours after some of Ukraine’s allies sought to rally new support for the embattled country. On a visit with leaders from three other EU countries on Russia’s doorstep who fear they could next be in Moscow’s sights, Lithuanian President Gitanas Nauseda declared that “the fight for Europe’s future is happening here.”

Meanwhile, U.S. President Joe Biden, who called Russia’s actions in Ukraine “a genocide” this week, approved $800 million in new military assistance to Kyiv. He said weapons from the West have sustained Ukraine’s fight so far and “we cannot rest now.”

The news of the flagship’s damage overshadowed Russian claims of advances in the southern port city of Mariupol, where they have been battling the Ukrainians since the early days of the invasion in some of the heaviest fighting of the war — at a horrific cost to civilians.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Wednesday that 1,026 troops from the Ukrainian 36th Marine Brigade 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.”

It was unclear when or over what time period a surrender may have occurred or how many forces were still defending Mariupol.

Russian state television broadcast footage that it said was from Mariupol showing dozens of men in camouflage walking with their hands up and carrying others on stretchers. One man held a white flag.

Mariupol’s capture is critical for Russia because it would put a swath of territory in its control that would allow its forces in the south, who came up through the annexed Crimean Peninsula, to link up with troops in the eastern Donbas region, Ukraine’s industrial heartland and the target of the coming offensive.

Moscow-backed separatists have been battling Ukraine in the Donbas since 2014, the same year Russia seized Crimea. Russia has recognized the independence of the rebel regions in the Donbas.

But the loss of the Moskva, which fires missiles, could set those efforts back.

Satellite photos from Planet Labs PBC show the Moskva steaming out of the port of Sevastopol on the Crimean Peninsula on Sunday.

Maksym Marchenko, the governor of the Odesa region, across the Black Sea to the northwest of Sevastopol, said the Ukrainians struck the ship with two Neptune missiles and caused “serious damage.” Russia’s Defense Ministry said ammunition on board detonated as a result of a fire and that it was investigating the cause of the blaze.

The Neptune is an anti-ship missile that was recently developed by Ukraine and based on an earlier Soviet design. The launchers are mounted on trucks stationed near the coast, and, according to the Washington-based Center for Strategic and International Studies, the missiles can hit targets up to 280 kilometers (175 miles) away.

It was not clear if the ship was totally disabled, but even serious damage could be a major blow to Russia, which already saw its tank carrier Orsk hit late last month.

Hours after the damage to the ship was reported, Ukrainian authorities said on the Telegram messaging service that explosions had struck Odesa, Ukraine’s largest port. They urged residents to remain calm and said there is no danger to civilians.

Russia invaded on Feb. 24 with the goal, according to Western officials, of rapidly seizing Kyiv, toppling the government and installing a Moscow-friendly replacement. But the ground advance stalled in the face of strong Ukrainian resistance with the help of Western arms, and Russia has lost potentially thousands of fighters. The conflict has killed untold numbers of Ukrainian civilians and forced millions more to flee.

A U.N. task force warned that the war threatens to devastate the economies of many developing countries that are facing even higher food and energy costs and increasingly difficult financial conditions. U.N. Secretary-General Antonio Guterres said the war is “supercharging” a crisis in food, energy and finance in poorer countries that were already struggling to deal with the COVID-19 pandemic, climate change and a lack of access to funding.

The war has also unsettled the post-Cold War balance in Europe — and particularly worried countries on NATO’s eastern flank that fear they could next come under attack. As a result, those nations have been some of Ukraine’s staunchest supporters.

The presidents of Poland, Lithuania, Latvia and Estonia traveled Wednesday to war-ravaged areas in Ukraine and demanded accountability for what they called war crimes. They met with Ukrainian President 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.

“There are no doubts that they committed war crimes. And for that, they should be accountable,” Latvian President Egils Levits said.

Nauseda of Lithuania called for tougher sanctions, including against Russian oil and gas shipments and all the country’s banks.

Ireland’s Foreign Minister Simon Coveney, who is also defense minister, visited Kyiv today.

In his nightly address, Zelenskyy noted that the prosecutor of the International Criminal Court visited the Kyiv suburb of Bucha, which was controlled by Russian forces until recently and where evidence of mass killings and more than 400 bodies were found.

“It is inevitable that the Russian troops will be held responsible. We will drag everyone to a tribunal, and not only for what was done in Bucha,” Zelenskyy said late Wednesday.

He also said work was continuing to clear tens of thousands of unexploded shells, mines and trip wires left in northern Ukraine by the departing Russians. He urged people returning to homes to be wary of any unfamiliar objects and report them to police.