Russia's guided missile cruiser Moskva has sunk in the Black Sea after being damaged during the military operation in Ukraine, Russia's defence ministry said.

Ukraine said the vessel was hit in a missile strike yesterday, while Russia described it as an explosion of ammunition on board, which caused a large fire.

The loss of the Moskva, the flagship in Russia's Black Sea fleet, occurred as it was being towed to port in stormy weather, Russian news agencies quoted the defence ministry as saying.

The sinking was a fresh blow to Moscow's military campaign as it readied for new assaults in east and south Ukraine that were likely to define the conflict's outcome.

Russia's defence ministry said earlier that over 500 crew aboard the Soviet-era missile cruiser were evacuated after ammunition exploded.

It did not acknowledge an attack and said the incident was under investigation.

Ukraine said it hit the warship with a Ukrainian-made Neptune anti-ship missile. The reports have not been verified.

The incident came as Russia's navy continues its bombardment of Ukrainian cities on the Black Sea nearly 50 days after it launched the invasion.

Residents of Odessa and Mariupol, on the adjacent Azov Sea, have been bracing for new Russian attacks.

The sinking of the Moskva was a "big blow" to Russia's naval strength in the Black Sea, the Pentagon said.

"This is a big blow to the Black Sea fleet, this is... a key part of their efforts to execute some sort of naval dominance in the Black Sea," Pentagon spokesman John Kirby told CNN.

"This is going to have an effect on their capabilities."

Kirby said the United States could not confirm the exact cause of the ship's demise.

"We're also not in any position to refute the Ukrainian side of this," he added. "It's certainly plausible and possible that they did in fact hit this with a Neptune missile or maybe more."

Latest Ukraine stories

Russia's warning over Finland and Sweden joining NATO

Russian forces have pulled back from some northern parts of Ukraine after suffering heavy losses and failing to take the capital Kyiv.

Ukraine and its Western allies say Moscow is redeploying for a new offensive in the eastern Donbas region.

Russia launched its assault in part to dissuade Ukraine from joining NATO. But the invasion has pushed Finland, which shares a long border with Russia, and nearby Sweden to consider joining the US-led military alliance.

Moscow warned NATO today that if Sweden and Finland join, Russia would deploy nuclear weapons and hypersonic missiles in a Russian enclave on the Baltic Sea, in the heart of Europe.

Commenting on Russia's military setbacks, CIA Director Williams Burns said the threat of Russia potentially using nuclear weapons in Ukraine cannot be taken lightly, but that the agency has not seen much practical evidence reinforcing that concern.

'Massing troops'

Ukrainian Deputy Defence Minister Hanna Malyar said in televised comments that Russia was massing troops not only along the Russia-Ukraine border, but also in Belarus and Moldova's breakaway Transdniestria region.

Authorities in Transdniestria, which borders southern Ukraine, had earlier in the week denied Russia was preparing forces there to threaten Ukraine.

The Kharkiv, Donetsk and Zaporizhzhia regions in the country's east were being hit by missile strikes, Ms Malyar said.

Kharkiv's governor said four civilians had been killed by shelling.

Russian authorities said villages in its southern regions of Bryansk and Belgorod had come under fire from Ukraine.

Neither side's statements could be independently verified and Ukraine's military did not respond to requests for comment about cross-border shelling.

Russia said yesterday that more than 1,000 Ukrainian marines from one of the scattered units still holding out in the shattered city of Mariupol had surrendered. Ukrainian officials did not comment.

If taken, Mariupol, Ukraine's main Sea of Azov port, would be the first major city to fall to Russian forces since they invaded on 24 February.

Its capture would allow Russia to reinforce a land corridor between separatist-held eastern areas and the Crimea region it seized and annexed in 2014.

Ruins of buildings destroyed by Russian shelling in Borodyanka, Kyiv region, Ukraine

'Forgive us'

Ukrainian Deputy Prime Minister Iryna Vereshchuk said nine humanitarian corridors had been agreed on for today to evacuate civilians, including by private car, from Mariupol.

Ukraine says tens of thousands of people are believed to have been killed in the city.

Mariupol's mayor, Vadym Boichenko, said Russia had brought in mobile crematoria "to get rid of evidence of war crimes" - a statement that it was not possible to verify independently.

Moscow has blamed Ukraine for civilian deaths and accused Kyiv of denigrating Russian armed forces.

The Kremlin says it launched a "special military operation" to demilitarise and "liberate" Ukraine from nationalist extremists, a message villagers said had been repeated to them by the Russian troops.