The Russian foreign ministry warned Friday of "retaliatory measures" over the entry of a U.S. naval destroyer in the Black Sea this week, the RIA news agency reported.
The USS Porter arrived in the Black Sea port of Varna, Bulgaria, on Tuesday for what the U.S. Navycalled a routine deployment. According to a statement from U.S. 6th Fleet, the destroyer "will conduct port visits and bilateral exercises with partner navies."
Its operations aim "to enhance maritime security and stability, readiness, and naval capability with our allies and partners," the statement added.
U.S. Navy officials on Wednesday also noted the deployment of two U.S. aircraft carriers, the Eisenhower and the Truman, in the Mediterranean Sea this month ahead of a NATO summit in Warsaw in July.
The beefed-up U.S. presence in the area rankled Russian officials.
"From time to time, U.S. vessels enter the Black Sea. Obviously, we do not appreciate it and, undoubtedly, this will lead to retaliatory measures," Andrei Kelin, a senior Foreign Ministry official, told the state-run RIA Novosti. He called the move on the eve of the NATO summit a "show of force."
Russian officials are increasingly unhappy over the increase in NATO activity in the region, particularly in former Soviet republics like Estonia, and former Warsaw Pact countries like Poland.
"As regards the overall situation, of course there is a definite increase and stoking of tensions in our relations," Kelin was quoted as saying.
Tensions between Washington and Moscow increased in the past two years over Russian intervention in the Crimea and Ukraine, on the Black Sea, and stepped-up Russian military activity in Syria.
The U.S. military also complained over what it said were risky air maneuvers by Russian aircraft over NATO ships during exercises last month in the Baltic.
U.S. presence in the Black Sea, where Russia maintains a major naval base, has been a longtime irritant for Moscow. However, NATO believes it's important to make clear the area is open to international traffic.
President Recep Tayyip Erdogan of Turkey, a NATO ally, said last month he complained to NATO Secretary General Jens Stoltenberg that the alliance needed to do more in the Black Sea to prevent Russian dominance in the area, the U.S. Naval Institute reported.
"I told him 'You are absent from the Black Sea,' " Erdogan said in an address to Balkan military leaders. "The Black Sea has almost become a Russian lake. If we don't act now, history will not forgive us."
Under the terms of the 1936 Montreux Convention, non-Black Sea countries can have no more than 9 warships in the Black Sea at one time and each ship must leave after 21 days.