On a day that saw Moscow suffer a stinging symbolic defeat with the loss of its Black Sea fleet flagship, Ukraine’s president hailed his people for their resolve since Russia invaded in February and for making “the most important decision of their life — to fight.”

In his nightly address, Volodymyr Zelenskyy told Ukrainians late Thursday that they should be proud of having survived 50 days under Russian attack when the invaders “gave us a maximum of five.”

Back then even friendly world leaders urged him to leave, unsure whether Ukraine could survive, he said: “But they didn’t know how brave Ukrainians are, how much we value freedom and the possibility to live the way we want.”

Listing the ways Ukraine has defended against the onslaught, Zelenskyy noted “those who showed that Russian warships can sail away, even if it’s to the bottom” of the sea.

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It was his only reference to the guided-missile cruiser Moskva, named for the Russian capital, which became a potent target of Ukrainian defiance in the opening days of the war. It sank Thursday while being towed to port after suffering heavy damage under circumstances that remained under dispute.

Ukrainian officials said their forces struck the vessel with missiles, while Moscow acknowledged a fire on board but not any attack. U.S. and other Western officials could not confirm what caused the blaze. In any case, the loss was a symbolic defeat for Russia as its troops regroup for a renewed offensive in eastern Ukraine after retreating from much of the north, including the capital, Kyiv.

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.

If Ukraine carried out the attack, the Moskva likely represents the largest warship to be sunk in combat since the 1982 Falklands War, which saw a similar-sized cruiser called the ARA General Belgrano torpedoed by a British submarine, killing over 300 sailors on board.

The news about 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.

Dwindling numbers of Ukrainian defenders in Mariupol 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 carpeted the streets with bodies.

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 is likely to add more ground combat units soon. 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.

Maksym Marchenko, governor of the Odesa region, said Ukrainian forces struck the Moskva 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 and that the crew, usually numbering about 500, abandoned the vessel. It wasn’t clear if there were any casualties. In addition to the cruise missiles, the warship also had air-defense missiles and other guns.

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The Neptune is an anti-ship missile that was recently developed by Ukraine 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, can hit targets up to 280 kilometers (175 miles) away. That would have put the Moskva within range, based on where the ship 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.

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

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 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.