KIEV/LVIV, Ukraine: Powerful explosions were heard in Kiev early on Friday, and air raid sirens blared across Ukraine as residents braced for new Russian attacks after Moscow's lead warship in the Black Sea sank following a fire.

The explosions appeared to be among the most significant in Ukraine's capital region since Russian troops pulled back from the area earlier this month in preparation for battles in the south and east.

Ukraine claimed responsibility for sinking the Moskva, saying the Soviet-era flagship of Russia's Black Sea fleet was struck by one of its missiles. The vessel sank late on Thursday as it was being towed to port, Russia's defence ministry said. Read full story

Over 500 crew aboard the missile cruiser were evacuated after ammunition on board exploded, the ministry said, without acknowledging an attack. Ukraine says it hit the warship with a locally made Neptune anti-ship missile.

The ship's loss comes as Russia's navy continues its bombardment of Ukrainian cities on the Black Sea nearly 50 days after it launched the invasion. Residents of Odesa and Mariupol, on the adjacent Azov Sea, have been bracing for new Russian attacks.

READ MORE: Latest updates on the Ukraine-Russia crisis

Ukrainian President Volodymyr Zelenskiy alluded to the sunken warship in an early morning video address in which he warned of Russian intentions to target the eastern Donbas region, including Mariupol.

Zelenskiy paid homage to all "those who halted the progress of the endless convoys of Russian military equipment ... Those who showed that Russian ships can go ... down to the bottom."

There were no immediate reports of damage following the explosions reported in Kiev, Kherson in the south, the eastern city of Kharkiv and the town of Ivano-Frankivsk in the west. Ukrainian media reported electricity outages in parts of Kiev.

Air raid sirens went off in all regions of Ukraine just after midnight on Friday and continued blaring in the eastern regions of Luhansk and Zaporizhzhia even after those elsewhere went quiet, Ukrainian media said.

Reuters could not immediately verify the reports.

MOSKVA

Whatever the cause of the Moskva's loss, the episode is a setback for Russia. If Ukraine's assertion that it hit the ship in a missile strike turns out to be true, the attack will go down in history as one of the highest-profile naval attacks so far this century. 

Russia's defence ministry said it is investigating the cause of the fire on board. The United States said it did not have enough information to determine whether the Moskva was hit by a missile.

"(But) certainly, the way this unfolded, it's a big blow to Russia," said national security adviser Jake Sullivan.

Russian forces have pulled back from some northern parts of Ukraine after suffering heavy losses and failing to capture Kiev. Ukraine and its Western allies say Moscow is redeploying for a new offensive in the eastern Donbas region.

Russia launched its assault in part to dissuade Ukraine from joining NATO. But the invasion has pushed Finland, which shares a long border with Russia, and nearby Sweden to consider joining the U.S.-led military alliance. Read full story

Moscow warned NATO on Thursday that if Sweden and Finland join, Russia would deploy nuclear weapons and hypersonic missiles in a Russian enclave on the Baltic Sea, in the heart of Europe. Read full story

Commenting on Russia's military setbacks, CIA Director William Burns said the threat of Russia potentially using nuclear weapons in Ukraine cannot be taken lightly, but that the agency has not seen much practical evidence reinforcing that concern. 

BATTLE FOR MARIUPOL

Moscow describes its invasion as a "special military operation" aimed at demilitarising Ukraine. Kiev and its allies say Russia has launched an unprovoked war that has seen more than 4.6 million people flee abroad and killed or wounded thousands.

Russia's navy has fired cruise missiles into Ukraine and its Black Sea activities are crucial to supporting land operations in the south and east, where it is battling to seize full control of Mariupol.

Russia said on Wednesday that more than 1,000 Ukrainian marines from one of the units still holding out in Mariupol had surrendered. Ukrainian officials did not comment.

If taken, Mariupol would be the first major city to fall to Russian forces since they invaded, allowing Moscow to reinforce a land corridor between separatist-held eastern Donbas areas and the Crimea region it seized and annexed in 2014.

Ukraine said tens of thousands of people were believed to have been killed in Mariupol, where efforts were under way to evacuate civilians.

Russia's defence ministry said late on Thursday that 815 people had been evacuated from the city over the past 24 hours. Ukraine said that figure was 289.