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The Russian Defence Ministry says the missile cruiser Moskva, the flagship of its Black Sea fleet, sank as it was being towed to port in stormy weather after an explosion and fire, Russian news agencies reported on Thursday.
The defence ministry earlier said the Soviet-era ship had been badly damaged by the fire, which Ukraine said was a result of its missile strike.
Maksym Marchenko, the Ukrainian Governor of the region around the Black Sea port of Odesa, said the Moskva had been hit by two Ukraine-made Neptune anti-ship cruise missiles.
“Neptune missiles guarding the Black Sea caused very serious damage,” he said in an online post.
A Pentagon official said the ship had an explosion while about 60 nautical miles south of the Ukrainian port city of Odesa.
Hours later, the Moskva was still “battling a fire on board”, even as the ship was moving east to the Crimean port of Sevastopol for repairs.
“We cannot confirm what caused the damage to the cruiser Moskva. We do believe that she has experienced significant damage,” the senior US defence official said.
Russia's Defence Ministry had said that a fire on the Moskva missile cruiser caused ammunition to blow up, Interfax news agency reported. It did not say what caused the fire.
The ministry earlier said there was no “open fire” after the ammunition blew up. Russian state media said the crew had left the ship and there were no immediate casualty reports.
Western security officials were unable to confirm that Ukraine-designed prototype Neptune missiles, which have a range of 315 kilometres, were responsible.
Ship-destroying missiles from Britain have yet to arrive in Ukraine.
A western military official said the loss of the Moskva would be a “massive blow to Russian” credibility.
“This is an enormous loss regardless of how it's happened; whether it's as a consequence of ineptitude on board or an attack by Ukrainian forces,” he said.
The incident either demonstrated that Russian warships were now “vulnerable” to Ukraine missiles or it was caused by incompetence, which would be a “blow to the sense of pride in Russia’s military”.
The Moskva is the second major ship known to have sustained serious damage since the start of the war. Last month, Ukraine said it had destroyed a landing support ship, the Orsk, on the Sea of Azov.
Russia's navy has launched cruise missiles into Ukraine and its activities in the Black Sea are crucial to supporting land operations in the south of the country, where it says it has seized full control of the port of Mariupol.
Defence experts have told The National that, despite being the flagship of the Black Sea Fleet, the Moskva was old, having been commissioned in 1983, and had had several maintenance upgrades cancelled.
The loss of the Moskva could ease the blockade on Ukraine’s Black Sea ports, allowing the country to export the mountains of grain desperately needed around the world, particularly in the Middle East.
Russian news agencies said the Moskva was armed with 16 anti-ship Vulkan cruise missiles with a range of at least 700km.
What is the Russian warship 'Moskva'?
Built in the Soviet era in Ukraine's Mykolaiv, the Moskva entered service in the early 1980s, Russian media report.
With a crew of 510, it was used in the Syria conflict, where it served as naval protection for the Russian forces' Hmeimim airbase.
The missile cruiser carries 16 P-1000 Vulkan anti-ship missiles and an array of anti-submarine and mine-torpedo weapons, the reports said.
The Moskva gained notoriety early in the war when its captain called on Ukrainian border troops defending the strategic Snake Island to surrender. They refused.
The troops were initially believed to have been killed, but were taken captive.
They were released as part of a prisoner exchange with Russia in late March, the Ukrainian Parliament said.
Ukraine's human rights ombudsman, Lyudmyla Denisova, said the soldiers described being taken to an unknown location where they were held in freezing conditions and suffered frostbite.