NATO warplanes intercepted a "significant" force of six Russian nuclear-capable bombers over the Baltic Sea, the alliance said in the latest such encounter amid tensions with Moscow over Ukraine.

Four Tu-95 Bear strategic bombers and two Tu-22M Backfire long-range bombers were intercepted in international airspace by Portuguese F-16 fighter jets assigned to NATO, spokeswoman Oana Lungescu said.

Lungescu said that while most intercepts carried out over the Baltic are "routine," the intercept of the six bombers "represented a significant level of activity by Russia."

In two other incidents later on Sunday, NATO warplanes intercepted a total of seven Russian military aircraft, including Tu-134s used for training and moving passengers as well as An-72 transport aircraft, Lungescu said.

"Such activity can be destabilising and potentially dangerous, if international norms are not respected," Lungescu warned.

She was referring to times when Russian warplanes do not communicate their positions or provide flight plans, putting at risk commercial airliners flying in the vicinity.

NATO has reported a surge in such intercepts since the crisis in Ukraine erupted earlier this year, at levels not seen since the Cold War between the Soviet Union and the West.

Alliance chief Jens Stoltenberg said there have been around 400 intercepts of Russian military flights near its member countries this year, 50 percent more than last year.

The US-led military alliance has deployed more aircraft as well as more ships and personnel in a bid to reassure newer members like the Baltic states and Poland, which were once ruled from Moscow and are unnerved by Russia's intervention in Ukraine.