WASHINGTON • A United States Air Force reconnaissance plane was intercepted by a Russian Su-27 jet in an "unsafe and unprofessional" manner while in international airspace over the Baltic Sea, the Pentagon said.

"The US aircraft was operating in international airspace and at no time crossed into Russian territory," spokesman Laura Seal said on Saturday of the Thursday incident.

It came after Russian aircraft repeatedly buzzed the USS Donald Cook last week - in an incident on Tuesday, a Russian Su-24 flew 9m above the warship in a "simulated attack profile", according to the US military's European Command.

Russia has denied the actions were reckless or provocative, but they have been seen as aggravating tensions between the two powers.

"This unsafe and unprofessional air intercept has the potential to cause serious harm and injury to all aircrews involved," Ms Seal said of the latest incident in a statement. "More importantly, the unsafe and unprofessional actions of a single pilot have the potential to unnecessarily escalate tensions."

The US aircraft in question was an RC-135, and the Pentagon said it had been flying a routine route.

"There have been repeated incidents in the last year where Russian military aircraft have come close enough to other air and sea traffic to raise serious safety concerns, and we are very concerned with any such behaviour," the Pentagon said.

US Secretary of State John Kerry said of the warship fly-by: "We condemn this kind of behaviour. It is reckless. It is provocative. It is dangerous. And under the rules of engagement, that could have been a shoot-down."

The Russian manoeuvres began on Monday while the destroyer was located about 70 nautical miles from the Russian base in Kaliningrad, a Russian enclave on the Baltic Sea.

AGENCE FRANCE-PRESSE