Three Ukrainian sea drones attacked two Russian Black Sea navy ships 340 km (211 miles) southwest of Sevastopol and were destroyed, TASS cited Russia's defence ministry as saying on Tuesday.
The ministry said the patrol ships were in the area to control navigation.
Russia has said it would treat any ships leaving or entering Ukrainian ports as valid targets after the expiration of the Black Sea grain deal last month which allowed for exports of Ukrainian grains.
The incident was the first clash of Ukrainian sea drones with the Russian navy in deep sea. Kyiv has previously used drones to target Russia's navy base in Crimea and the bridge that Russia has built to the peninsula.
The Russian defence ministry said the Sergei Kotov and Vasily Bykov ships continued to perform their missions in the Black Sea.
