Ukraine carried out a fiery missile strike on Friday on the main headquarters of Russia's Black Sea Fleet, a Russian official said.
Videos and photos showed large plumes of smoke over the building in Sevastopol in annexed Crimea.
The Russian-installed governor of Sevastopol, Mikhail Razvozhayev, said there was no information about casualties, but firefighters were battling a blaze, and more emergency forces were being brought in, an indication that the fire could be massive.
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A stream of ambulances was arriving at the fleet's headquarters and shrapnel was scattered hundreds of metres around, Russia's Tass news agency reported.
Razvozhayev initially warned Sevastopol residents that another attack was possible and urged them not to leave buildings or go to the city centre.
He later said there was no longer any air strike danger but reiterated calls not to go to the central part of the city, saying roads were closed and and unspecified "special efforts" were underway.
Ukrainian officials, who have claimed responsibility for a series of other recent attacks on Crimea, didn't immediately announce Kyiv launched the strike.
Sevastopol residents said they heard explosions in the skies and saw smoke, Russian news outlets reported.
Images circulated in Ukrainian Telegram channels showed clouds of smoke over the seafront. The Associated Press could not immediately verify the videos.
The attack comes a day after Russian missiles and artillery pounded cities across Ukraine, killing at least five people as President Volodymyr Zelenskyy met with President Joe Biden and congressional leaders in Washington with an additional $24 billion aid package being considered.
The port city of Sevastopol serves as the main base for Russia's Black Sea Fleet.
Last week, the Russian-installed authorities there accused Ukraine of attacking a strategic shipyard in the city, damaging two ships undergoing repairs and causing a fire at the facility.
The Crimean Peninsula, which Russia annexed from Ukraine in 2014 in an act that most of the world considered illegal, has been a frequent target since Russian President Vladimir Putin ordered a full-scale invasion of Ukraine more than 18 months ago.
The attack on the shipyard was the biggest in weeks.
In other developments, ongoing shelling in the southern Kherson region killed one man and injured another, said regional Governor Oleksandr Prokudin.
"Kherson has been restless since the morning," he said on Telegram.
Russian shelling sparked fires in a residential building and a garage.
In Kharkiv, regional Gov. Oleh Synyehubov said over 14 settlements came under attack.
A house was damaged and a fire broke out in Vovchansk, in the Chuguyiv district. There were no casualties, the governor said.
