The flagship of Russia's Black Sea Fleet has sunk, Moscow has claimed after the warship was damaged following an explosion.
Russian media said Moskva, which Ukraine claimed it struck with missiles, was being towed back to port when it sank.
It comes after an earlier report from the Russian defence ministry that the Soviet-era ship had been badly damaged by fire.
Russian defence officials said ammunition on board the 186-metre (610 foot) vessel had exploded in an unexplained fire, but Ukraine claimed it struck the ship with its Neptune missiles.
Oleksiy Arestovych, an adviser to Ukraine’s president Volodymyr Zelensky, called it an event of “colossal significance.”
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Russia denied there was an attack on the warship and said that its guided missile launchers were intact.
The loss of the ship would be a huge military and symbolic defeat for Russian president Vladimir Putin. However, neither statement by the two sides about the incident has been independently verified.
Western officials later claimed, however, that Ukraine’s claim of a missile strike appeared “credible” - describing the Kremlin’s version of events as “difficult to believe”.
“We don’t have the capacity at this point to independently verify that but certainly, the way this unfolded, it’s a big blow to Russia,” said US national security adviser Jake Sullivan.
Moskva, which would usually house a crew of around 500 sailors, was some 69 miles south of the port city of Odesa when the fire broke out, according to an unnamed senior US defence official.
The Moskva‘s loss deals a fresh blow to the Russian offensive as it prepares for a new assault in the eastern Donbas region that is likely to define the conflict’s outcome.
Commenting on Russia’s setbacks, CIA Director William Burns said the threat of Russia potentially using tactical or low-yield nuclear weapons in Ukraine cannot be taken lightly, but the CIA has not seen a lot of practical evidence reinforcing that concern.
During the early stages of the war in February, a group of Ukrainian soldiers posted to Snake Island, known as Zmiyyny, in the Black Sea, refused to surrender, telling an approaching Russian warship - the Moskva - to “go f*** yourself”.
Reports of damage to Russia’s flagship earlier today took precedent over Moscow’s claims of advances in Mariupol, the southern port city which has been subject to some of the most brutal fighting of the war - at a horrific cost to civilians.
A spokesman for Russia‘s defence ministry, major general Igor Konashenkov, earlier claimed that 1,026 Ukrainian troops 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”.
It was unclear how many forces were still defending Mariupol.
Its capture is of critical importance to Vladimir Putin’s forces as it would allow Russian troops in the south to join up with those in the eastern Donbas region, Ukraine’s industrial heartland and the target of the coming offensive.
Although it still remains unclear as to when Russia could launch a bigger offensive in the Donbas, the loss of the Moskva has the potential to delay any new, wide-ranging assault.
