MOSCOW, Jan. 19 (Xinhua) -- Russia's Black Sea fleet is closely monitoring a U.S. missile destroyer in the sea, the Russian National Defense Control Center said Saturday.

"Units of the Black Sea Fleet started tracking the USS Donald Cook, a guided missile destroyer, immediately after it entered the Black Sea," the center was quoted as saying by Interfax news agency.

The U.S. vessel entered the Black Sea at 8.50 p.m. Moscow time (1750 GMT), the center said.

The Pytlivy guard ship is watching actions of the U.S. ship, using electronic and technical surveillance means during the whole period of the ship's presence in the fleet's responsibility zone, it said.

According to existing regulations, the U.S. warship, which belongs to a country without a Black Sea coastline, is not allowed to stay in the sea longer than 21 days, the center was quoted by Tass news agency as saying.

Earlier on the day, the U.S. Navy said that the ship started heading into the Black Sea to conduct maritime security operations.

Russia started inspecting commercial ships sailing through the Sea of Azov and the Kerch Strait after inaugurating a bridge over the strait.

On Nov. 25, Russia seized three Ukrainian naval ships and 24 sailors on board attempting to sail through the Kerch Strait from the Black Sea to the Sea of Azov.