A Russian Sukhoi Su-27 fighter in Baltic airspace. Photo: Reuters/RAF

Stockholm: NATO accused Russia of posing a danger to civil aviation in the Baltic region after Stockholm protested over a Russian air force jet it said had flown too close to an airliner and had turned off one of its location instruments.

Friday's incident off southern Sweden inflamed sensitivities over Russian air force flights in the Nordic region that have increased steeply this year, driven in part by tensions over Moscow's role in the Ukraine crisis. Finland also expressed concern about "Dark Flights" with so-called transponder locators switched off.

"It is not only a question of increased...flights but it's the way they're conducting the flights. They are not filing their flight plans and they are not communicating with civilian air traffic control and they are not turning on their transponders," NATO Secretary General Jens Stoltenberg told a news conference on Monday.

"That poses a risk to civilian air traffic. The important thing is that NATO stays vigilant and that we intercept the Russian flights."

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Russia denied its aircraft had posed any hazard to the airliner.

The Swedish military said the Russian jet had turned off its transponder - a communications device, alongside normal radar, that makes it easier for an airplane to be located, especially in congested air space.

While civilian flights must fly with their transponders on at all times, military flights are allowed to turn them off when flying in international air space as long as they show consideration to other flights.

A NATO spokesman said NATO aircraft always kept their transponders turned on when flying in European air space.

Sweden and Denmark said they had summoned Russian ambassadors over the behaviour of the military aircraft which Swedish authorities said had caused an SAS flight from Copenhagen to Poznan, Poland, to change course.

Finland's government instructed air safety authorities and ministry officials on Sunday to contact Russian colleagues, and said it wanted "dark flights" - with transponders turned off - to be discussed at the International Civil Aviation Organization.

"There is no direct defence policy threat against Finland, but looking at the current flight activity above the Baltic Sea, one could say that the situation is...much more tense than in a long, long time," Prime Minister Alexander Stubb said in a radio interview with YLE radio.

NATO said earlier this month that its aircraft had scrambled more than 400 times this year to intercept Russian aircraft, up 50 percent from the 2013 total.

Reuters