Russia's flagship in the Black Sea has been damaged after being struck by missiles, a Ukrainian official has said this morning.
Unconfirmed reports say that 300 Russian sailors who were aboard may have died.
The Kremlin claims that the Moskva missile cruiser was seriously damaged after ammunition on board the ship exploded but Ukraine says that it was struck by two Neptune missiles, The Mirror reports.
There are conflicting reports from Ukrainian sources if the ship has been sunk.
The Telegram channel, Ishchi Svoikh, which was set up by the Ukrainian Interior Ministry to tell the relatives of the Russian forces about their troops, reported the cruiser sank at roughly 3am local time.
Shortly before that, the Russian Ministry of Defence claimed the crew had evacuated but Ukrainian officials say that is "far from being the case."
The message reads: "An effective rescue operation could not be carried out due to a storm, about 300 sailors died.“
A Ukraine official said the missile attack was the biggest blow to the Russian navy since World War 2.
Russa's defence ministry denies reports it sunk and has claimed it has contained the fire, that the cruiser's main weaponry had not been damaged and that the crew had been evacuated onto other ships.
It added that an operation was underway to tow it back to the port.
The Neptune missile is a coastal anti-cruise ship missile that is capable of destroying naval ships in a range of 300km, according to the Ukrainian Ministry of Defence.
Anton Gerashchenko, an advisor to the Ukrainian ministry, said: “Usually, in cases of an explosion of ammunition on such a large surface ship between a quarter and half crew perishes.
"There were 510 [crew] on the Moskva. This means that between 125 to 300 died. Mothers of Russia, wait for the funeral and thank Putin."
Elsewhere, Maksym Marchenko, governor of the region around the Black Sea port of Odesa, triumphantly claimed in an online post that the 12,500-tonne ship was hit by two missiles.
"Neptune missiles guarding the Black Sea caused very serious damage," he said in an online post.
Michael Petersen, director of the Russia Maritime Studies Institute at the US Naval War College, told the Radio 4's Today programme about the news that the Russian vessel the Moskva has been seriously damaged.
He says that it is too early to know what happened but it appears to look like the "Ukrainians have managed to pull off a not-insignificant victory at sea".
Mr Petersen said: "I think it's one that has symbolic and military value as well.
"This is the flagship of Russia's Black Sea fleet, and it's getting a bit long in the tooth but it is the symbol of Russian naval power in the Black Sea."
He went on to explain that the Ukrainians had lost most of their navy to the Russians during the annexation of Crimea in 2014, and that this attack was "important symbolically".
He continued: "Because after all of that, they are still able to strike at Russian military power in ways that will really hurt back in Moscow."
He adds the "Russian navy may feel obligated to operate further off-shore now than they were previously".
"That may affect its ability to provide direct fire support to troops ashore and it might hinder their ability to provide air defence in coastal regions... I do think it will make the Russian navy think twice."
The 600 feet flagship has been used by the Russians as a show of strength in the Black Sea.
The Moskva was the ship that fired on 13 defiant border guards on Snake Island early on in the invasion where one of the Ukrainians famously said "go f*** yourselves" after they were told to surrender.
Ukrainian presidential advisor Oleksiy Arestovych, reportedly said the ship was "burning strongly" in a YouTube video
He stated: "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. We don't understand what happened."
Last month Ukraine said it had destroyed a large Russian landing support ship, the Orsk, on the smaller Sea of Azov to the north east of the Black Sea. Moscow has not commented on what had happened to the ship.
It comes as Ukraine has warned that Russia is ramping up attempts in the south and east to take full control of Mariupol, in what would be the first major city to fall.
Western governments are sending more military aid to bolster Kyiv.
Russia's defence ministry on Wednesday said 1,026 soldiers from Ukraine's 36th Marine Brigade, including 162 officers, had surrendered in Mariupol, which has been besieged for weeks, and that the port was fully under its control.
Capturing its Azovstal industrial district, where the marines have been holed up, would give the Russians full control of Ukraine's main Sea of Azov port, reinforce a southern land corridor and expand its occupation of the country's East.
Ukraine's general staff said Russian forces were attacking Azovstal and the port, but a defence ministry spokesman said he had no information about any surrender.
"Russian forces are increasing their activities on the southern and eastern fronts, attempting to avenge their defeats," President Volodymyr Zelensky said in a Wednesday night video address.
Flames were reportedly seen billowing from the Azovstal area on Tuesday, a day after Ukraine's 36th Marine Brigade said its troops had run out of ammunition.
The United States has announced an extra $800 million in military assistance including artillery systems, armoured personnel carriers and helicopters, taking total US military aid to more than $2.5 billion. France and Germany also pledged more.
Senior US officials are weighing whether to send a top cabinet member such as Secretary of State Antony Blinken and Defence Secretary Austin Lloyd to Kyiv in a show of solidarity, a source familiar with the situation said.
Russia will view US and NATO vehicles transporting weapons on Ukrainian territory as legitimate military targets, Deputy Foreign Minister Sergei Ryabkov told the TASS news agency.
It will impose tit-for-tat sanctions on 398 members of the US House of Representatives and 87 Canadian senators, Interfax cited the foreign ministry as saying, after Washington targeted 328 members of Russia's lower house of parliament.
Ukraine says tens of thousands of people are believed to have been killed in Mariupol and accuses Russia of blocking aid convoys to civilians marooned there.
Moscow has blamed Ukraine for civilian deaths and accused Kyiv of denigrating Russian armed forces.
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