A Russian landing ship has been destroyed and two other boats have been damaged in the occupied Ukrainian port city of Berdyansk, say Ukrainian officials.
The Ukrainian military posted footage early on Thursday and said the Orsk had been hit by its forces.
Details of what caused the explosion and fire on board the ship are unclear.
Berdyansk, which is west of the besieged port of Mariupol, was seized four days after Russia invaded Ukraine.
Russia says it has used the port as a base to ferry in equipment for its troops.
Russian army TV hailed the arrival of the Orsk in Berdyansk last week as an “epic event” as it was the first Russian warship to dock there.
Drone footage filmed by a Russian state TV reporter showed an armoured personnel carrier being offloaded from the Orsk in the port. The armoured vehicles were to be used to reinforce Russian troops, the TV report added.
Video posted by the navy and on social media showed explosions and a big ship on fire at the port at 07:00 (05:00GMT) on Thursday.
Deputy Defence Minister Hanna Malyar told Ukrainian TV that the military had hit a “huge target”, capable of carrying 20 tanks, 45 armoured vehicles and 400 troops.
The BBC cannot independently verify the claim.
Berdyansk is of major strategic value to the Russians, lying between Crimea and the besieged city of Mariupol, some 80km (50 miles) to the east, where an estimated 100,000 people remain stranded facing Russian bombardment.
Capturing the towns of Berdyansk and Melitopol are part of Russia’s bid to create a land bridge from Crimea to the Russian border, as well as establishing an inland route towards the city of Zaporizhzhia.
Residents have staged protests against Russia’s occupation of Berdyansk, and video posted on social media this week showed the military beating people in an attempt to disperse the crowd.
Ukrainian officials said the attack on the Orsk had further symbolic value, suggesting that it may have taken part in Russia’s original capture of Crimea in 2014, pointing to medals apparently awarded to the ship’s commander and a crewman.