A big Russian amphibious ship, the Caesar Kunikov, has sunk off the coast of Russian-occupied Crimea according to Ukraine's armed forces.
Powerful explosions were heard early on Wednesday morning, according to local social media, which suggested the landing ship had been hit not far south of the town of Yalta.
Ukraine has repeatedly hit Russia's Black Sea fleet in occupied Crimea.
Satellite images last year showed much of the fleet had left the peninsula.
Ukraine's Main Directorate of Intelligence published a video of what it said were Magura V5 naval drones striking the landing ship.
Russia's full-scale war in Ukraine is due to enter its third year next week and Ukraine's armed forces chief has admitted the situation is "extremely complex".
Col Gen Oleksandr Syrskyi, who was appointed commander-in-chief last week, visited the front line on Wednesday hours after a Russian missile attacked claimed three lives in the eastern town of Selydove.
There was no confirmation from Russia's navy that the Caesar Kunikov had been sunk in the Black Sea, merely that six Ukrainian drones had been destroyed. Video appearing to show the aftermath of the Ukrainian attack was uploaded only recently, BBC Verify confirmed.
"The Caesar Kunikov suffered critical holes in its port side and began to sink," Ukraine's intelligence directorate said on the Telegram messaging site, adding that it had been destroyed in Ukrainian territorial waters by a unit called Group 13.
Amphibious ships are used to move assault troops to land quickly, especially in enemy territory. Russia has also used landing ships in recent years to ferry military supplies to Syria, in support of Bashar al-Assad's government.
Russian military bloggers did not deny the Caesar Kunikov had been hit, saying only that the crew had survived. Russia's military rarely reports major losses and Russians rely on a handful of popular bloggers for information.
The Caesar Kunikov dates back to the end of the Soviet era. If its sinking is confirmed, it would be the second successful strike in the Black Sea this month. A small warship, the Ivanovets, was hit by drones in a special operation almost two weeks ago.
Another Russian landing ship, the Novocherkassk, was hit while in port in Feodosiya in December.