Russia said on Thursday the flagship of its Black Sea fleet was seriously damaged and its crew evacuated following an explosion that a Ukrainian official said was the result of a missile strike.
Russia's defence ministry said a fire on the Moskva missile cruiser caused ammunition to blow up, Interfax news agency reported.
It did not say what caused the fire but Maksym Marchenko, the Ukrainian governor of the region around the Black Sea port of Odesa, said the Moskva had been hit by two Ukrainian-made Neptune anti-ship cruise missiles.
"Neptune missiles guarding the Black Sea caused very serious damage," he said in an online post.
Ukraine's defence ministry did not respond to a request for comment and Reuters was unable to verify either side's claims.
The Moskva is the second major ship known to have suffered serious damage since the start of the war. Last month Ukraine said it had destroyed a landing support ship, the Orsk, on the smaller Sea of Azov.
Russia's navy has launched cruise missiles into Ukraine and its activities in the Black Sea are crucial to supporting land operations in the south of the country, where it is battling to seize full control of the port of Mariupol.
Russian news agencies said the Moskva, commissioned in 1983, was armed with 16 anti-ship Vulkan cruise missiles with a range of at least 700 km.
Russia said 1,026 soldiers from Ukraine's 36th Marine Brigade, including 162 officers, had surrendered in Mariupol and that the city was fully under its control. Ukraine's defence ministry spokesman said he had no information about a surrender.
Capturing the Azovstal industrial district where the marines have been holed up would give Russia control of Ukraine's main Sea of Azov port, reinforce a southern land corridor and expand its occupation of the country's east.
"Russian forces are increasing their activities on the southern and eastern fronts, attempting to avenge their defeats," President Volodymyr Zelenskiy said in a Wednesday night video address.
Reuters journalists accompanying Russian-backed separatists saw flames billowing from the Azovstal area on Tuesday, a day after Ukraine's 36th Marine Brigade said its troops had run out of ammunition.
The United States said on Wednesday it would send an extra $800 million worth of military hardware to Ukraine including artillery, armoured personnel carriers and helicopters. France and Germany also pledged more.
Senior US officials are weighing whether to send a top cabinet member such as Secretary of State Antony Blinken and Defense Secretary Austin Lloyd to Kyiv in a show of solidarity, a source familiar with the situation said.
Russia will view US and NATO vehicles transporting weapons on Ukrainian territory as legitimate military targets, Deputy Foreign Minister Sergei Ryabkov told the TASS news agency.
It will impose tit-for-tat sanctions on 398 members of the US House of Representatives and 87 Canadian senators, Interfax cited the foreign ministry as saying, after Washington targeted 328 members of Russia's lower house of parliament.
Britain announced new financial measures on separatists, and Australia imposed targetted financial sanctions on 14 Russian state-owned enterprises on Thursday.
Fiji said it was investigating the arrival of the superyacht Amadea, owned by Russian billionaire Suleiman Kerimov, who has been sanctioned by the United States, Britain and the European Union.
BIDEN'S GENOCIDE COMMENTS
The Kremlin denounced President Joe Biden's description of Moscow's actions in Ukraine as amounting to genocide, with spokesman Dmitry Peskov saying this was unacceptable coming from the leader of a country he said had committed crimes of its own.
An initial report by a mission of experts set up by the Organization for Security and Cooperation in Europe documents a "catalogue of inhumanity" by Russian troops in Ukraine, according to the US ambassador to the OSCE.
"This includes evidence of direct targetting of civilians, attacks on medical facilities, rape, executions, looting and forced deportation of civilians to Russia," Michael Carpenter said.
Russia has denied targetting civilians.
The Kyiv district police chief said 720 bodies had been found in the region around the capital from where Russian forces had retreated, with more than 200 people missing.