The Russian Navy flagship missile cruiser 'Moskva' (right) docked in the bay of the Crimean city of Sevastopol. AFP

The Russian navy's Black Sea flagship has been "seriously damaged" by an ammunition explosion, Russian state media said on Thursday.

"As a result of a fire, ammunition detonated on the Moskva missile cruiser. The ship was seriously damaged," the Russian defence ministry was quoted as saying, adding that the cause of the fire was being determined and that the crew had been evacuated. Meanwhile, a Ukrainian official said the fire was the result of a missile strike.

Russia's defence ministry said a fire on the Moskva missile cruiser caused ammunition to blow up, Interfax news agency reported.

READ MORE

Russia has yet to slow a Western arms express into Ukraine

Over 1000 Ukrainian soldiers surrender in Mariupol: Russia

Earlier, the governor of Odessa said that Ukrainian forces had hit Moskva with missile strikes.

It did not say what caused the fire but Maksym Marchenko, the Ukrainian governor of the region around the Black Sea port of Odesa, wrote on Telegram, "Neptune missiles guarding the Black Sea caused very serious damage to the Russian ship. Glory to Ukraine!"

This file photo shows the Moskva guided missile cruiser near a navy base in Sevastopol. AFP

Ukrainian presidential adviser Oleksiy Arestovych said that "a surprise happened" with Moskva.

"It burns strongly. Right now. And with this stormy sea, it is unknown whether they will be able to receive help," he said in a YouTube broadcast.

"We don't understand what happened."

Russia's navy has launched cruise missiles into Ukraine and its operations in the Black Sea are crucial to supporting land operations in south and east Ukraine.

Russian news agencies said the Moskva, commissioned in 1983, was armed with 16 anti-ship "Vulkan" cruise missiles with a range of at least 700 km (440 miles).

Ukraine warned that Russia was ramping up efforts in the south and east as it tried to seize full control of the besieged port of Mariupol.

The Moskva was originally built in the Soviet era in Ukraine's Mykolaiv, and entered service in the early 1980s, according to Russian media.

With a crew of 510, it was previously deployed 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 as well as an array of anti-submarine and mine-torpedo weapons, the reports said.

The Moskva gained notoriety early in the war when it called on Ukrainian border troops defending the strategic Snake Island to surrender, only to be defiantly refused.