Ukraine's military says it attacked and destroyed a Russian submarine while it was anchored at a port in the occupied Crimean peninsula.
The Rostov-on-Don, a kilo-class attack submarine launched in 2014, sank after it was struck in a missile attack on the port city of Sevastopol on Friday, Ukraine's general staff said in a statement.
It was reportedly one of four submarines operated by Russia's Black Sea fleet capable of launching Kalibr cruise missiles. The Russian defence ministry has not commented.
Officials in Kyiv said the attack also destroyed four S-400 air defence systems protecting the peninsula, which Russia illegally annexed in 2014.
Intelligence officials in the UK noted last September that the Rostov-on-Don "likely suffered catastrophic damage" in a missile strike while undergoing maintenance at a Sevastopol shipyard.
Ukraine's military said Russia subsequently repaired the vessel and it was recently testing its capabilities near Sevastopol. The vessel was worth $300m (£233m), they added.
"The destruction of Rostov-on-Don once again proves that there is no safe place for the Russian fleet in the Ukrainian territorial waters of the Black Sea," the general staff in Kyiv said in a statement on Saturday.
It marks the latest attack on Russian naval forces in Sevastopol in recent months. In March alone, Ukraine said it hit two landing ships and a patrol vessel in the port city.
Since Russia launched its unprovoked invasion of Ukraine in February 2022 it has suffered several major naval setbacks. Ukraine says it has seriously damaged or sunk at least 15 warships, including the Black Sea fleet's flagship, the Moskva.
Last week Ukraine's military said Moscow had been forced to withdraw all of its naval assets from the Sea of Azov - a body of water connected to the Black Sea - due to repeated strikes on its vessels.
And Russia's internal security service, the FSB, recently said it foiled a Ukrainian plot to destroy its last remaining aircraft carrier, the Admiral Kuznetsov. The ship, launched in 1985, has been undergoing repairs since 2018.
Meanwhile, officials in Kyiv said Ukrainian drones targeted a major airfield and oil depots in Russia.
The attack targeted the Morozovsk airfield, where guided bombs which have recently wrecked havoc on Ukrainian cities, are stored.
Online footage said to be from the base showed powerful explosions and huge fires, after what appears to be several hits on fuel or ammunition depots. Russia said many of the drones used were shot down, but local authorities have declared a state of emergency around the air base.
Oil storage facilities were also targeted in the Rostov, Kursk and Belgorod regions.
The attacks come after Russia launched more than 600 guided air bombs towards Ukraine in a week, according to President Volodymyr Zelensky.
He said that it was crucial that Ukraine stopped Russian aircraft from launching the munitions and said that attacking airfields in Russia to do so was "quite fair".
Ukraine's allies have previously been reluctant to allow it to strike within Russia using Western weapons, though the US has recently granted Kyiv permission to attack some targets along the border.
Earlier this week Lithuania's foreign minister said the first deliveries of F-16 fighter jets had arrived in Ukraine. Long promised by Kyiv's Nato allies, President Zelenky views the planes as central to his country's air defence plans.
The Times newspaper reported that six jets donated by the Netherlands had arrived in the country, but Dutch defence officials declined to comment when approached by the OceanNewsUK earlier this week.
Officials in Kyiv will also hope that the jets can help arrest Russian momentum on the frontlines. Moscow's forces have been making incremental gains in the east of the country for several weeks.
