The Pentagon on Monday described last week's "unsafe" manoeuver by a Russian fighter that flew close to a US spy plane, saying it had performed a barrel roll.

Pentagon spokesman Navy Captain Jeff Davis said a Russian Su-27 flew less than 50 feet (15 meters) from the wingtip of an RC-135 reconnaissance plane in international airspace over the Baltic Sea last Thursday.

It then "conducted a barrel roll from the left side of the aircraft, going over the top of the aircraft, and ended up on the right side of the aircraft," Davis said.

Thursday's "unsafe and unprofessional" incident came just days after Russian aircraft repeatedly buzzed the USS Donald Cook in a series of incidents decried by the US military.

In one case, a Russian Su-24 flew 30 feet (nine meters) above the war ship in a "simulated attack profile," according to the US military's European Command.

Ties between Russia and the West have plunged to their lowest point since the Cold War over Moscow's 2014 annexation of the Crimean peninsula from Kiev and its support for separatist rebels in eastern Ukraine.

It did not appear that the Russian jets were armed in any of the incidents, the Pentagon said.