A Russian official said the incident would not ease tensions
The US Air Force has released images showing a Russian fighter jet which had been scrambled to intercept American bombers.
An SU-27, known by its NATO reporting name of "Flanker", was deployed to head off a US B-52H craft, capable of carrying a nuclear weapon, during exercises taking place over the Baltic Sea.
It also intercepted two B-1B Lancers and a KC-135R Stratotanker.
The alliance is currently running its annual Baltic Operations (BALTOPS) exercise, a multinational, maritime-focused live training event which first began in 1972, in the region close to Russia's border.
A Russian foreign ministry official said the appearance of the B-52H would not ease tensions between the West and Moscow.
The ministry said the SU-27 approached at a safe distance, identified the aircraft as an American B-52 strategic bomber, and escorted it until it changed course and flew away from the border area.
The Russian government-controlled news agency Sputnik quoted Mikhail Ulyanov, the director of the Russian Foreign Ministry, as stating that a number of the strategic bombers had been relocated to the UK to take part in the BALTOPS exercises.
Mr Ulyanov said: "The fact that NATO forces are converging near Russia's borders and carrying out military exercises supported by strategic bombers from the USA capable of carrying nuclear weapons hardly helps de-escalate tensions in Europe."
A statement from the US military claimed the exercise was taking place in international airspace.
"We can confirm that the US Air Force B-52 was operating in international airspace but we don't have any information to provide at this time regarding the behavior of Russian aircraft," Air Force spokesman Colonel Patrick Ryder said.