Moscow's defence ministry said the Moskva missile cruiser was still afloat and its main weaponry had not been damaged.

Ukrainian officials, however, said the vessel had sunk, describing it as a "resounding slap in the face" for Moscow.

Neither side's claims have been independently verified.

The Moskva's crew was evacuated on to other ships in the area, Russia's defence ministry said, admitting earlier that it was badly damaged.

"As a result of a fire on the Moskva missile cruiser, ammunition detonated," it said in comments reported by the Tass news agency.

The cause of the fire is being investigated, the ministry added.

It said later: "The source of the blaze on the Moskva has been contained. The explosions of ammunition have stopped."

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A 'symbolic' event on 'both sides'

Air Marshall Phil Osborn, former chief of UK defence intelligence, said the Ukrainian version of events - that the fire was caused by a missile strike - was "more likely" to be correct.

The development is "quite symbolic on both sides", he told Sky News, adding that the Moskva is "quite a significant vessel as far as Russia is concerned".

Presuming the cruiser was indeed hit by a Ukrainian missile, the impact is "less the loss of the ship and more a demonstration of Ukrainian capability", Air Marshall Osborn added.

The Moskva, which dates from the Soviet era, was commissioned 40 years ago but has since been refitted, Mr Osborn said.

A missile strike could cause the Russians to "stand off a bit", he commented, which may have "real implications for any potential assault on Odesa".

'She is on fire'

Two Ukrainian sources told Sky News the 12,500-tonne ship had been hit by anti-ship missiles launched by the Ukrainian military.

"She is on fire," one of the sources said. "The level of damage is being clarified… She is about 25 nautical miles from Snake Island."

Two Neptune missiles were used, Ukrainian media reported.

Again, those claims have not been independently verified.

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Maksim Marchenko, the governor of Odesa, said on Telegram: "It has been confirmed that the missile cruiser Moskva today went exactly where it was sent by our border guards on Snake Island!

"Neptune missiles guarding the Black Sea caused very serious damage to the Russian ship.

"Glory to Ukraine!"

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His comments about Snake Island refer to an incident earlier in the war - the day after Russia's invasion began - when the Moskva was one of two Russian ships approaching the patch of land in the Black Sea.

The Russians ordered 13 Ukrainian soldiers defending the island to surrender but the Ukrainians responded over radio, telling the Moskva and its crew to "go f*** yourself".

At first it was thought the Ukrainian soldiers had been killed but the country's navy later said they had been captured alive by Russia.

Other key developments:

• Russia threatens nuclear escalation if Sweden and Finland join NATO
• US announces $800m in military support for Ukraine
• US officials consider sending Secretary of State Antony Blinken or Defence Secretary Lloyd Austin to Kyiv
• The presidents of Poland, Lithuania, Latvia, and Estonia visit Ukraine to meet President Volodymyr Zelenskyy

Ukraine has warned that Russia is boosting its efforts in the south and east as it edges closer to taking control of Mariupol.

Russia's defence ministry said on Wednesday that 1,026 soldiers from Ukraine's 36th Marine Brigade, including 162 officers, had surrendered in the southern city and that the port had come under Russian control.

Ukrainian soldiers trying to defend the strategically-important city have been fighting for weeks while running out of food, water, and ammunition.

Ukraine's general staff confirmed Russian forces were attacking the port area but said there was no information about a surrender.