The Russian cruiser Moskva, the 12,000 tonne flagship of its Black Sea fleet, has exploded and efforts are underway to save its crew. 

The Russian Defence Ministry has confirmed the heavily modernised Soviet-era guided missile cruiser suffered "significant damage" after "a fire broke out" and set off its ammunition. 

It says it is "investigating" the cause of the explosion. 

The admission came after Ukraine's governor for the port of Odesa claimed two R-360 "Neptune" class anti-ship missiles had been fired at the largest warship in Russia's Black Sea fleet. 

He claimed the Moskva had exploded and sunk. 

Ukrainian presidential advisor Oleksiy Arestovych said that "a surprise happened with the flagship of the Russian Black Sea Fleet". 

"It burns strongly. Right now. And with this stormy sea it is unknown whether they will be able to receive help. There are 510 crew members," he said in a YouTube broadcast. "We don't understand what happened." 

The Kremlin says the command ship's 500 crew was "completely evacuated". 

Open-source intelligence analysts reported picking up distress signals from the warship and its companions being transmitted in morse code. The signals included notifications such as "SOS, "sinking" and attempts to rescue its crew. 

The Moskva was operating in poor weather conditions at the time of the attack. Unconfirmed reports suggest the warship was "distracted" by a Ukrainian drone as it was being targeted by the shore-based anti-ship missiles. It was unable to redirect its defences in time to fend off the attack. 

The Moskva was the imposing warship involved in the famous standoff from the opening days of the Ukrainian war. The handful of border guard defenders of Snake Island responded to radio demands that they surrender with the retort: "Russian warship, go f**k yourself!" 

Highlighted are the large supersonic anti-ship missiles that make the ship so vulnerable to explosions. Picture: Russian Ministry of Defence 

The island's defenders were later captured and repatriated by Russian forces. 

Moskva is a Project 1164 Slava class guided missile cruiser. Its design follows a Russian philosophy of packing as many large weapons on the hull as possible, making it particularly susceptible to devastating explosions. 

The Slava class was originally built to shadow and destroy US navy nuclear-powered aircraft carriers. It carries 16 large supersonic anti-ship missiles strapped to its side for this purpose. 

It also has a significant anti-aircraft missile system. This may be why the ship was operating off the coast of Odesa – supporting and protecting the blockade of this important port. 

The loss of the Moskva means Russia has no similar-sized warship left in the region. 

The Montreux Convention, which controls passage through Turkey's Dardanelles and Bosporus Channel between the Black Sea and the Mediterranean means Russia cannot send replacement warships from its other fleet bases. 

Naval affairs analyst H.I. Sutton recently noted the Russian flagship had fallen into a "predictable pattern" of behaviour. 

"Using open-source intelligence (Osint), we have pieced together her areas of operation during the war so far," he wrote. "Although we only have periodic snapshots, patterns have emerged." 

Sutton says Russian warships began conducting intimidation missions close to Odesa early in March, effectively creating a blockade. "We have not seen any evidence that Moskva took an active part. Instead, she remained further offshore," he notes. "with Open Source Intelligence, we are able to track her much of the time." 

- by Jamie Siedel, news.com.au