Russia’s flagship warship, a Black Sea missile cruiser, the Moskva, has sunk in rough seas while being towed after suffering a fire onboard, according to Russian media.
The defence ministry said ammunition on the Moskva exploded in an unexplained fire on Wednesday.
Ukraine claims it struck the 86-metre (610 foot) vessel with its Neptune missiles.
The 510-crew warship has led Russia’s naval assault on Ukraine, making it an important symbolic and military target.
Earlier in the conflict the Moskva gained notoriety after calling on Ukrainian border troops defending Snake Island in the Black Sea to surrender – to which they memorably radioed a message of refusal which loosely translates as “go to hell”.
Originally built in Ukraine in the Soviet-era, the sunk Russian warship entered service in the early 1980s according to Russian media.
The missile cruiser was previously deployed by Moscow in the Syria conflict where it supplied Russian forces in the country with naval protection.
The cruise missile system that Ukraine says was deployed was designed by Ukrainian military engineers in response to the growing naval threat posed by Russia in the Black Sea, following its annexation of Crimea in 2014.
As our Russia editor, Steve Rosenberg, writes: Whatever the cause of the Moskva’s destruction, the damage is bad news for Moscow.
It is the flagship of Russia’s Black Sea fleet, and, as such, a symbol of Russia’s military power.
Casualty from the sunk Russian warship was not immediately available.