In a statement posted on the Telegram messaging app, Russia’s defence ministry said Ukraine had attempted what it called “a terrorist attack on Russian civilian transport ships using semi-submersible naval drones in the southwestern part of the Black Sea”.

It said one Ukrainian naval drone involved in the attack had been destroyed by artillery fire and the rest disabled by electronic warfare. No Russian civilian or military vessels were damaged, it said.

Reuters could not verify the Russian account. There was no immediate comment from Ukraine.

The Black Sea is a vital route for Russian and Ukrainian grain and oil exports. Last July, Russia refused to renew a Turkish-brokered deal providing for safe grain exports through the region.

It was not clear precisely where the attack took place. The southwestern part of the Black Sea adjoins Turkey’s Bosphorus Strait through which cargoes leaving the sea travel.

Ukraine has in recent months mounted a series of drone and missile attacks on Russian military targets in the Black Sea, sinking at least one naval vessel and damaging others.