Ukraine said it struck a submarine and several other targets in the city of Sevastopol early on Wednesday, in what appeared to be the largest attack of the war on the home of the Russian Black Sea Fleet.
A large landing vessel and port infrastructure were among the sites hit in the attack on Russian-annexed Crimea.
"We confirm a large landing vessel and submarine were hit. We do not comment on the means (used) for the strike," Ukrainian military intelligence official Andriy Yusov said.
Russia's defence ministry said in statement that Ukraine had attacked a Black Sea shipyard with 10 cruise missiles and three uncrewed speedboats in the early hours of Wednesday, damaging two vessels that had been undergoing repairs.
It said it downed seven of the incoming missiles and that the attack boats had been destroyed by a Russian patrol ship.
"It really is the biggest attack on Sevastopol since the beginning of the war," said retired Ukrainian navy captain Andriy Ryzhenko.
Mikhail Razvozhayev, the Moscow-appointed governor of Sevastopol, said on Telegram on Wednesday that 24 people were injured in the fire at the shipyard. He posted a photo of the blaze with smoke billowing.
The attack came as Moscow launched drones overnight against the Odesa region in southern Ukraine.
Drone strikes damaged port and civilian infrastructure in Odesa's Izmail district and wounded six people, including three seriously, regional governor Oleh Kiper said.
The Ukrainian Air Force said it intercepted 32 of 44 Shahed-type drones launched over Ukraine overnight, with most aimed towards southern parts of Odesa.
The Sevastopol attack appeared to one of the biggest in recent weeks, even though the Crimean peninsula, annexed from Ukraine by Russia in 2014, has been a frequent target in the 18-month war.