The Moskva, part of Russia’s Black Sea fleet, sank after being hit by two Neptune missiles off the coast of Odesa, Ukrainian defence chiefs said.
Russia confirmed the ship was damaged but claimed ammunition on board detonated after a fire in the early hours of Thursday morning.
It is believed to be the first missile cruiser to be sunk during a conflict since the Falklands War.
Western officials said that while it had not been independently established whether Ukrainian weapons had hit the warship, it was a show of “incompetence” that it had been destroyed.
“In terms of credibility for Russian forces it’s an enormous loss regardless of how it’s happened,” they said.
“Whether it’s as a consequence of ineptitude on board or an attack by Ukrainian forces. Regardless of how it’s happened, it’s a massive blow for Russian credibility. I think that’s why we’re seeing conflicting Russian narratives in that space.
“Because either they’ve been vulnerable to an attack by Ukrainians, and that questions their competence, or they have had a fire on board a capital ship, which has then resulted in detonation of its magazine, where its ammunition is stored, and that is just another bit of incompetence.”
They added if Ukraine was responsible for the attack then it shows the “imagination and initiative” of Volodymyr Zelensky’s forces and exposes “another example of Russian military failure”.
Odesa’s governor Maksym Marchenko said about 300 Russian sailors are believed to have died in the strike on the Moskva.
Russia claims the entire crew were evacuated from the ship stationed about 25 miles off Snake Island.
A Telegram channel run by the Ukraine defence ministry said: “The cruiser Moskva sank at about 3am. Shortly before this, the Russian Ministry of Defence announced that there was a fire on the ship, the ammunition detonated, the crew was evacuated.
“But according to our data, this is far from being the case. An effective rescue operation could not be carried out due to a storm, about 300 sailors died.”
The Moskva was seen as a key component in Russia’s plan to take the Odesa region.
The ship leads Putin’s Slava class — a fleet of guided missile cruisers constructed in the Soviet era and now at the heart of the Russian navy.
Photos show the heavily modernised Moskva leaving Sevastopol port in Crimea on April 10.
The vessel was reported to have issued an SOS just after 1am local time.
Analyst Sergey Mardan, who works for the pro-Kremlin KP radio, posted on his Telegram social media channel: “Moskva cruiser sank.”