Russia’s Black Sea flagship Moskva missile carrier has sunk after an explosion and fire that Ukraine claimed was a successful missile strike.

The Moskva missile cruiser had been leading Russia’s naval effort in the seven-week conflict.

Russia’s defence ministry said the blast on the vessel was the result of exploding ammunition and added that the resulting damage had caused it to “lose its balance” as it was being towed to port on Thursday.

“Given the choppy seas, the vessel sank,” the Russian state news agency TASS quoted the ministry as saying.

On the Ukrainian side, Odesa military spokesman Sergey Bratchuk said the ship had been hit by domestic Neptune cruise missiles.

Russia earlier said the flames on the ship, which would typically have 500 sailors on board, forced the entire crew to evacuate. It later said the blaze had been contained.

The Moskva could carry 16 long-range cruise missiles, and its removal reduces Russia’s firepower in the Black Sea. It is also a blow to Moscow’s prestige in a war already widely seen as a historic blunder.

In his nightly video address to the nation, President Volodymyr Zelenskyy alluded to the sinking as he told Ukrainians they should be proud of having survived 50 days under attack when the Russians “gave us a maximum of five”.

Listing the many ways Ukraine has defended against the invasion, he noted “those who showed that Russian warships can sail away, even if it’s to the bottom” of the sea. It was his only reference to the missile cruiser.

The news of the flagship overshadowed Russian claims of advances in the southern port city of Mariupol where Moscow’s forces 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.

On Wednesday, Russian defence ministry spokesman Igor Konashenkov said 1,026 Ukrainian soldiers 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”.

The strength of the forces still defending Mariupol was unclear.

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 has been the scene of some of the war’s worst sufferings.

Dwindling numbers of Ukrainian defenders are holding out against a siege that has trapped well beyond 100,000 civilians in desperate need of food, water and heating.

Mariupol’s mayor said this week that more than 10,000 civilians had died and the death toll could surpass 20,000, after weeks of attacks and privation left bodies “carpeted through the streets”.

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