The flagship of Russia's Black Sea fleet, a guided-missile cruiser that became a potent target of Ukrainian defiance in the opening days of the war, sank Thursday after it was heavily damaged in the latest setback for Moscow's invasion.

Ukrainian officials said their forces hit the vessel with missiles, while Russia acknowledged a fire aboard the Moskva but no attack. U.S. and other Western officials could not confirm what caused the blaze.

The loss of the warship named for the Russian capital is a devastating symbolic defeat for Moscow as its troops regroup for a renewed offensive in eastern Ukraine after retreating from much of the north, including the capital, Kyiv.

In his nightly video address to the nation, President Volodymyr Zelenskyy alluded to the sinking as he told Ukrainians they should be proud of having survived 50 days under attack when the Russians “gave us a maximum of five.”

Listing the many ways Ukraine has defended against the invasion, he noted “those who showed that Russian warships can sail away, even if it’s to the bottom” of the sea. It was his only reference to the missile cruiser.

The Russian Defense Ministry said the ship sank in a storm while being towed to a port. Russia earlier said the flames on the ship, which would typically have 500 sailors aboard, forced the entire crew to evacuate. Later it said the blaze had been contained.

Also read: Why the term ‘genocide’ matters in Ukraine war

The Moskva had the capacity to carry 16 long-range cruise missiles, and its removal reduces Russia’s firepower in the Black Sea. It's also a blow to Moscow's prestige in a war already widely seen as a historic blunder. Now entering its eighth week, the invasion has stalled amid resistance from Ukrainian fighters bolstered by weapons and other aid sent by Western nations.

During the first days of the war, the Moskva was reportedly the ship that called on Ukrainian soldiers stationed on Snake Island in the Black Sea to surrender in a standoff. In a widely circulated recording, a soldier responded: “Russian warship, go (expletive) yourself.”

The Associated Press could not independently verify the incident, but Ukraine and its supporters consider it an iconic moment of defiance. The country recently unveiled a postage stamp commemorating it.

The news of the flagship overshadowed Russian claims of advances in the southern port city of Mariupol, where Moscow's forces have been battling the Ukrainians since the early days of the invasion in some of the heaviest fighting of the war — at a horrific cost to civilians.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Wednesday that 1,026 Ukrainian troops surrendered at a metals factory in the city. But Vadym Denysenko, adviser to Ukraine’s interior minister, rejected the claim, telling Current Time TV that “the battle over the seaport is still ongoing today.”

It was unclear how many forces were still defending Mariupol.

Russian state television broadcast footage that it said was from Mariupol showing dozens of men in camouflage walking with their hands up and carrying others on stretchers. One man held a white flag.

Also read: Polish, Baltic presidents visit Ukraine in show of support

Mariupol has been the scene of the some the war's worst suffering. Dwindling numbers of Ukrainian defenders are holding out against a siege that has trapped well over 100,000 civilians in desperate need of food, water and heating. David Beasley, executive director of the U.N. World Food Program, told AP in an interview Thursday that people are being “starved to death” in the besieged city.

Mariupol’s mayor said this week that more than 10,000 civilians had died and the death toll could surpass 20,000, after weeks of attacks and privation left bodies “carpeted through the streets.”

Mariupol's capture is critical for Russia because it would allow its forces in the south, which came up through the annexed Crimean Peninsula, to fully link up with troops in the Donbas region, Ukraine's eastern industrial heartland and the target of the coming offensive.

The Russian military continues to move helicopters and other equipment together for such an effort, according to a senior U.S. defense official, and it will likely add more ground combat units “over coming days.” But it’s still unclear when Russia could launch a bigger offensive in the Donbas.

Moscow-backed separatists have been battling Ukraine in the Donbas since 2014, the same year Russia seized Crimea. Russia has recognized the independence of the rebel regions in the Donbas.

The loss of the Moskva could delay any new, wide-ranging offensive.

Maksym Marchenko, the governor of the Odesa region, across the Black Sea to the northwest of Sevastopol, said the Ukrainians struck the ship with two Neptune missiles and caused “serious damage.”

Russia’s Defense Ministry said ammunition on board detonated as a result of a fire, without saying what caused the blaze. It said the “main missile weapons” were not damaged. In addition to the cruise missiles, the warship also had air-defense missiles and other guns.

The Neptune is an anti-ship missile that was recently developed by Ukraine and based on an earlier Soviet design. The launchers are mounted on trucks stationed near the coast, and, according to the Washington-based Center for Strategic and International Studies, the missiles can hit targets up to 280 kilometers (175 miles) away. That would have put the Moskva within range, based on where it was when the fire began.

Launched as the Slava in 1979, the cruiser saw service in the Cold War and during conflicts in Georgia and Syria, and helped conduct peacetime scientific research with the United States. During the Cold War, it carried nuclear weapons.

In 1989, the Slava was supposed to host a meeting off Malta between Soviet leader Mikhail Gorbachev and U.S. President George H.W. Bush, but gale-force winds moved the talks to the docked cruiser Maxim Gorky.

On Thursday, other Russian ships that were also in the northern Black Sea moved further south after the Moskva caught fire, said a senior U.S. defense official who spoke on condition of anonymity to discuss internal military assessments.

Before the Moskva sank, Yuriy Sak, an adviser to Ukraine’s defense minister, told AP its removal would mean “we can only have a sigh of relief.”

While the U.S. was not able to confirm Ukraine’s claims of striking the warship, U.S. national security adviser Jake Sullivan called it “a big blow to Russia."

“They’ve had to kind of choose between two stories: One story is that it was just incompetence, and the other was that they came under attack, and neither is a particularly good outcome for them,” Sullivan told the Economic Club of Washington.

Russia invaded on Feb. 24 and has lost potentially thousands of fighters. The conflict has killed untold numbers of Ukrainian civilians and forced millions more to flee.

It has also further inflated prices at grocery stores and gasoline pumps, while dragging on the global economy. The head of the International Monetary Fund said Thursday that the war helped push the organization to downgrade economic forecasts for 143 countries.

Also Thursday, Russian authorities accused Ukraine of sending two low-flying military helicopters some 11 kilometers (7 miles) across the border and firing on residential buildings in the village of Klimovo, in Russia's Bryansk region. Russia’s Investigative Committee said seven people, including a toddler, were wounded.

Russia’s state security service had earlier said Ukrainian forces fired mortar rounds at a border post in Bryansk as refugees were crossing, forcing them to flee.

The reports could not be independently verified. Earlier this month, Ukrainian security officials denied that Kyiv was behind an air strike on an oil depot in the Russian city of Belgorod, some 55 kilometers (35 miles) from the border.