Swedish defense officials have said a Russian military aircraft nearly collided south of Sweden with a commercial passenger plane shortly after takeoff from Copenhagen, Denmark, on December 12.
Sweden's air force chief, Major General Micael Byden, said the Russian warplane's transponders -- which make the plane detectable to commercial radar -- were shut off.
Swedish Defense Minister Peter Hultqvist called the incident "serious" and "inappropriate," although he added the Russian plane had not violated Swedish air space.
"This is outright dangerous when you turn off the transponder," Hultqvist told Swedish radio.
Hultqvist said both Danish and Swedish jet fighters were scrambled in response to the incident, which comes amid heightened tensions between the West and Russia over the conflict in Ukraine.
Russia's Defense Ministry denied any of its warplanes had violated Swedish airspace on December 12.
Quoted by TASS, Defense Ministry spokesman Major General Igor Konashenkov said the Russia aircraft's flight had been made "in compliance with international air space rules, not violating borders of other countries and at a safe distance from traffic routes of civilian aircraft."
The commercial plane was reportedly flying to Poland, but it was unclear which airline was operating the flight or how many passengers were on board.
In March, a Russian military jet flying without transponders came within 100 meters of an SAS plane that had taken off from Copenhagen.
Byden said the latest incident was not as serious as the near miss in March.
On December 11, Polish Defense Minister Tomasz Siemoniak said Russia's military is engaging in an unprecedented amount of activity around the Baltic Sea.
"Over the past few days we have seen unprecedented activity by the Russians in the Baltic Sea, both the Baltic fleet and Russian aircraft," Siemoniak told Polish broadcaster TVN24.
The minister said that Poland, a member of NATO, was not under threat of attack and the Russian maneuvers were most likely designed to test how NATO forces in the region reacted.
A report issued in November said the number and gravity of "close military encounters" between Russia and the West has increased visibly amid tension over Ukraine.
The report by the European Leadership Network detailed almost 40 dangerous or sensitive incidents over the past eight months, including what it says was the near-collision of a Russian spy plane and an SAS jet carrying 132 passengers from Copenhagen to Rome on March 3.
It says the incidents "add up to a highly disturbing picture" of airspace violations and other dangerous actions "over a very wide geographical area" from the Baltic and Black Seas to the U.S. and Canadian borders.