A Russian naval warship has sunk in the Black Sea, dealing a blow to Moscow’s war plans. The massive warship, called the Moskva, came to international attention early in the Ukraine war, when a small number of Ukrainian soldiers refused to surrender an island to the troops aboard. Reports differ about who is to blame for its sinking.

The Russian Defence Ministry initially refused to acknowledge that the warship had sunk. The first reports of its possible sinking came in the early hours of Thursday morning. Still, neither news outlets nor U.S. intelligence could verify whether it had sunk or just severely damaged by an explosion.

On Thursday afternoon, Russia’s defense ministry claimed the ship had been damaged but was still afloat before acknowledging Thursday night that it had sunk.

“During the towing of the Moskva cruiser to the port of destination, the ship lost its stability due to damage to the hull received during the fire from the detonation of ammunition. In the conditions of stormy seas, the ship sank,” the Russian Defence Ministry said.

Ukraine was quick to seize on the sinking of the Moskva as a sign of its military successes. Ukrainian officials claimed they had fired a Neptune anti-ship missile at the Moskva and damaged it. However, there has yet to be any footage released to support Ukrainian claims.

The few details that have been confirmed include that a large explosion damaged the Moskva’s hull. Ukraine claims its Neptune missile caused the damage.

Russia denies the Ukrainian claims, saying it was an accidental explosion caused by a fire that had ignited ammunition on board the ship.

Stormy weather hampered the rescue mission for the Moskva as four nearby vessels responded to the damage. According to the Russian Defence Ministry, more than 500 crewmembers were rescued from the ship.

Konstantin Zatulin, a senior Russian lawmaker, said some sailors had likely died in the explosion.

The Ukrainian version of events differs significantly from that given by Russia. Ukrainian officials said the crew of the Moskva was distracted by a drone, giving its Army the chance to fire the Neptune missile. After the explosion, the Moskva withdrew 20 nautical miles away from the coast before attempting to tow the ship into port. Ukraine claims the ship’s retreat was to avoid being hit by more Neptune missiles.

“It’s a hugely important military event and the biggest defeat of the Russian Navy since World War II,” Ukrainian presidential adviser Oleksiy Arestovych wrote on social media. He added that aside from the symbolism of its sinking, as “Moskva” is Russian for Moscow, this would mark the biggest warship by tonnage sunk since 1945.

The Ukrainian military has been asking for more anti-ship missiles over the last few weeks as Russian ships are being used in the bombing of towns and cities along the Black Sea Coast.

Russia’s remaining naval ships have retreated 80 nautical miles from the Black Sea Coast. A Neptune missile has a range of approximately 100 nautical miles.

The potential success of the Ukrainian military came as Russia turned its attention to Finland.

After Sweden and Finland began discussing becoming full members of NATO to protect themselves from Russian aggression, Russia responded by saying it would defend itself at all costs from NATO aggression, as it had in Ukraine.