Sweden and Denmark today summoned Russia's ambassadors to express their dismay at the latest incident, in which a passenger plane leaving Copenhagen on Friday was forced to change direction to avoid colliding with a Russian military jet. Danish Foreign Ministry spokesman Lars Peter Levy said the purpose of the meeting was to make clear that any activity that threatens passenger safety is unacceptable.

Russia has made repeated incursions into Nordic airspace and Baltic waters as the stand-off in Ukraine sours relations with its former Cold War foes. The Dec. 12 near-miss follows a similar incident in March, when a SAS AB flight almost hit a Russian reconnaissance plane. Such events carry a "high probability" of leading to casualties or direct military confrontation, according to a November report by the European Leadership Network, a think tank.

"It's very important that air security is taken seriously by all states, including Russia," Danish Defense Minister Nicolai Wammen said in a statement. "It's dangerous and completely unnecessary that Russian military planes fly so close to civilian air traffic in the Baltic Sea."

Russia is embroiled in its most serious confrontation with the U.S. and its European allies since the collapse of the Soviet Union in 1991. With tensions flaring over Ukraine, NATO jets have tracked Russian military planes over the Baltic and North seas and the Atlantic Ocean in recent weeks.

Russia Defense Ministry spokesman Igor Konashenkov denied the surveillance plane had broken international airspace rules or had endangered civilian aircraft. The Russian plane was 70 kilometers (45 miles) from the passenger plane, RIA Novosti cited him as saying.

According to Swedish authorities, Russian planes have been flying without their transponders on. Sweden's Prime Minister Stefan Loefven said today that this is "completely irresponsible."

"We need to think about what more international authorities can do, for example within flight safety, in order to sharpen the rules," he said. "We can't accept that someone would behave like this, fly without transponders."

Finland's air traffic control has in recent days also redirected civilian flights to avoid Russian military planes, newspaper Helsingin Sanomat reported, citing Pekka Henttu of the Finnish Transport Safety Agency's aviation sector.

"Passenger safety must always be the first priority," Denmark's Wammen said. "And Russia has to respect that."

Meanwhile, as the stand-off between Nordic countries and Russia intensifies, Denmark has decided to stake an official claim to a part of the Arctic circle, an area also being contested by Russia, as well as Norway and Canada. The Danish government said today it has "scientific documentation" that supports its claim to an area covering almost 900,000 square kilometers.