Moskva, the flagship of the Russian Navy's Black Sea Fleet, has sunk during the country's ongoing invasion of Ukraine.
Russia's defence ministry said it sank as it was towed back to port in stormy weather following an explosion.
Earlier, Russian officials claimed the damage was the result of a fire on board, while a Ukrainian official said it was a result of a Ukrainian missile strike.
The ship was thought to be located in the Black Sea somewhere off the Ukrainian port of Odesa at the time of the explosion.
The Moskva would be the biggest Russian warship damaged by enemy fire since 1941, when German dive bombers crippled Soviet battleship Marat in Kronshtadt harbour, according to military analysts.
Here is what we know so far about the ship and what may have happened to it.
What is Moskva, is it important?
Moskva has been one of Russia's most critical warships, first entering service with the Soviet navy in 1983.
It is a guided-missile cruiser and the flagship of the Russian Black Sea Fleet, which operates in the Black Sea, the Sea of Azov and the Mediterranean Sea.
Russian news agencies said Moskva was armed with 16 anti-ship Vulkan cruise missiles with a range of at least 700 kilometres.
The ship was known as Slava during its Soviet service, before taking the name Moskva (Russian for Moscow) in 2000.
It carried both surface-to-surface and surface-to-air missiles, deck guns, torpedoes and mortars. It also had a helicopter deck.
During Russia's war in its former republic of Georgia in 2008, the Moskva took part in operations in the Black Sea, and Georgia said it was involved in an attack on the country.
In 2014, as Russia annexed the Crimean Peninsula, the Moskva blocked Ukrainian naval vessels from leaving Lake Donuzlav.
In 2015-16, it was deployed to the Mediterranean Sea to provide support for the Russian military campaign backing Syrian President Bashar Assad in his country's civil war. Its sailors were decorated for their service there and in the war on Georgia.
The 12,500-tonne ship usually has a crew of about 500 people.
But some experts say the Moskva's loss is more symbolically damaging than anything practical to Russia's invasion in Ukraine.
"The ship is really very old. Actually, there have been plans to scrap it for five years now," Russian military analyst Alexander Khramchikhin said.
Ukrainian authorities said Moskva featured in one of the landmark early exchanges of the war in Ukraine, when Ukrainian border guards on Snake Island, a small outcrop in the Black Sea, told the ship to "go **** yourself" after it demanded they surrender.
Although Ukrainian officials initially said all 13 guards had died in the subsequent Russian attack, Ukrainian President Volodymyr Zelenskyy later said some of them survived.
How was Moskva damaged?
Russian officials said a fire on Moskva caused some ammunition to explode, the Interfax news agency reported.
"As the result of a fire on the Moskva missile cruiser, ammunition detonated. The ship was seriously damaged," the Russian Defence Ministry said in a statement.
"The crew was completely evacuated."
Officials added that the "causes of the fire were being established".
Russian news agencies later reported that Moskva sank in stormy weather as it was towed back to port.
If the damage was caused by some kind of an onboard explosion, it would be the second flagship of Russia's Black Sea Fleet to be taken out of action in a similar fashion.
The Imperatritsa Maria dreadnought battleship sank in 1916 after an explosion involving ammunition.
But Maksym Marchenko, the Ukrainian governor of the region around the Black Sea port of Odesa, claimed the Moskva had been hit by two Ukrainian-made Neptune anti-ship cruise missiles, which caused "very serious damage".
He did not provide evidence.
"It has been confirmed that the missile cruiser Moskva today went exactly where the border guards on Snake Island told it to go," he said in an online post.
There are unconfirmed reports that Moskva sank after sustaining this damage.
Ukraine's defence ministry did not respond to a request for comment, and the news agency Reuters was unable to verify either side's claims.
Alessio Patalano, a professor of war and strategy at King's College in London, told CNN that losing the warship would be a "massive blow" for Russia.
"Ships operate away from public attention and their activities are rarely the subject of news," he said.
"But they are large floating pieces of national territory and when you lose one, a flagship no less, the political and symbolic message — in addition to the military loss — stands out precisely because of it."
Moskva is second major Russian ship to be destroyed
Last month, Ukraine said it had also destroyed a Russian landing support ship, the Orsk, on the smaller 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.
Russia said 1,026 soldiers from Ukraine's 36th Marine Brigade, including 162 officers, had surrendered in the key port city of Mariupol and that the area was fully under its control.
Ukraine's defence ministry spokesman said he had no information about a surrender.
Capturing the Azovstal industrial district where the marines have been holed up would give Russia control of Ukraine's main Sea of Azov port, reinforce a southern land corridor and allow it to expand its occupation of eastern Ukraine.
