Ukrainian forces said they hit and badly damaged the flagship of Russia’s Black Sea fleet, and Russia said its crew was forced to evacuate as a result of a fire without acknowledging an attack, as the battle shifts east and around the battered city of Mariupol.

The governor of the Odesa region, Maksym Marchenko, said the Ukrainians struck the guided-missile cruiser Moskva with two missiles and caused “serious damage”.

Russia’s defence ministry confirmed the ship was damaged but not that it was hit by Ukraine – it said ammunition on board detonated as a result of a fire of as-yet undetermined causes.

The entire crew was evacuated, it added – the cruiser typically has about 500 on board.

If confirmed, the sinking of the cruiser would be a major blow to Russia, after the tank carrier Orsk was hit and set on fire in an attack in Berdyansk in the Sea of Azov late last month.

The reported ship attack by Neptune cruise missiles came a day after US president Joe Biden called Russia’s actions in Ukraine “a genocide” and approved 800 million dollars (£600 million) in new military assistance to Kyiv, saying weapons from the West have sustained Ukraine’s fight so far and “we cannot rest now”.

The munitions include artillery systems, armoured personnel carriers and helicopters.

The presidents of Poland, Lithuania, Latvia and Estonia showed their support for Ukraine in a visit on Wednesday to war-ravaged areas and demanded accountability for what they called war crimes.

They met their counterpart, Volodymyr Zelensky, 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.

The Lithuanian president, Gitanas Nauseda, added: “The fight for Europe’s future is happening here.”

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

In one of the most crucial battles of the war in the southern port city of Mariupol, Russian defence ministry spokesman Maj Gen Igor Konashenkov said 1,026 troops from the Ukrainian 36th Marine Brigade surrendered at a metals factory.

But Vadym Denysenko, adviser to Ukraine’s interior minister, told Current Time TV that “the battle over the seaport is still ongoing today”.

Russian troops are gearing up for a major offensive in the eastern Donbas region, where Moscow-allied separatists and Ukrainian forces have been fighting since 2014.

Mariupol is a key piece in the Russian campaign and lies in the Donbas, which the Russians have pummelled for weeks.

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

Russian state television broadcast footage Wednesday that it said was from Mariupol showing dozens of men in camouflage walking with their hands up and carrying others on stretchers or in chair holds.

One man held a white flag. In the background was a tall industrial building with its windows shattered and roof missing, identified by the broadcaster as the Iliich metalworks.

A UN taskforce 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.

UN 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 pandemic, climate change and a lack of access to funding.

In his nightly address, Mr Zelensky said 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,” Mr Zelensky said.