Ukrainian officials say they have caused “serious damage" to the flagship of Russia's Black Sea fleet, the guided-missile cruiser Moskva.
The Odesa region's governor Maksym Marchenko said forces struck the ship with two neptune anti-ship missiles.
Unofficial reports from Russian military experts said on Thursday that the Moskva cruiser had sunk.
The Russian Defence Ministry confirmed the ship was damaged, but not that it was hit by Ukraine. The ministry said ammunition on board detonated as a result of a fire whose causes “were being established", adding that the Moskva's entire crew was evacuated.
The Moskva is the flagship of his Black Sea Fleet and was seen as a key player in an expected bid to extend hostilities to Odessa. It is a guided missile cruiser, the lead ship of the Project 1164 Atlant class and was named after the city of Moscow.
A picture had shown the the Moskva leaving Sevastopol port in Crimea on April 10.
The vessel was reported to have issued an SOS about 1.05am local time on Thursday.
Analyst Sergey Mardan, with pro-Kremlin KP radio, posted on his Telegram channel: “Moskva cruiser sank”.
The incident was some 25 miles off Snake Island.
Earlier, President Joe Biden approved $800 million (£610m) in military assistance for Ukraine, including artillery and helicopters, to bolster its defences against a Russian offensive in the country's east.
Mr Biden announced the aid after a call with Ukrainian President Volodymyr Zelensky to coordinate the delivery of the assistance, which he said included artillery systems, artillery rounds and armoured personnel carriers, as well as helicopters.
"This new package of assistance will contain many of the highly effective weapons systems we have already provided and new capabilities tailored to the wider assault we expect Russia to launch in eastern Ukraine," Mr Biden said.
He added that the US will continue to work with allies to share additional weapons and resources as the conflict continues.
Mr Biden is under pressure from members of both parties in Congress to expand and accelerate US aid.
