A U.S. Navy guided-missile destroyer and a command and control ship sail in formation during the U.S.-Ukraine multinational maritime exercise Sea Breeze 2018 in the Black Sea July 13, 2018. (Reuters Photo)

MOSCOW, March 29 (Xinhua) -- Two Russian warships are escorting three frigates of the North Atlantic Treaty Organization (NATO) in the Black Sea, Russian media reported Friday.

The Ivan Khurs reconnaissance ship and the Vasily Bykov corvette of the Russia Black Sea Fleet are monitoring the movement of the NATO vessels, TASS news agency said, quoting the National Defense Management Center.

Turkish DNA agency reported earlier in the day that three frigates, Canadian Toronto, Spanish Santa Maria and Dutch Evertsen, have passed the Bosporus Strait on their way to the Black Sea.

The mission of the NATO naval group was not immediately clear.

The NATO has been conducting exercises with Georgia, located on the Black Sea, From March 18 to March 29, involving 343 military and civilian personnel from 21 NATO members, Azerbaijan, Sweden and Georgia.

NATO Secretary General Jens Stoltenberg expressed "concerns about Russia's military build-up in the Black Sea region" at a press conference on Monday in Georgia's capital of Tbilisi.

Stoltenberg said NATO has increased its presence in the Black Sea to address the "security challenges."

Russia has just completed drills in Crimea, involving more than 1,500 paratroopers, over 300 pieces of equipment, warplanes, helicopters and large landing ships, the Russian Defense Ministry said Friday.

NATO and Russian military maneuvers were conducted ahead of the Ukrainian presidential election on March 31, when incumbent President Petro Poroshenko will seek a re-election.