A Russian military jet nearly collided with a commercial passenger airplane in international airspace near southern Sweden on Friday, the Swedish authorities said, but Russia insisted on Sunday that its jet had kept at a safe distance.

Relations between Russia and the West have soured over Moscow's role in the conflict in Ukraine and its annexation of Crimea. Many European countries have reacted with alarm to suspected displays by Russia of its military prowess.

A squadron of Russian warships entered the English Channel last month and Sweden said it had proof a foreign submarine was operating illegally in its waters in October. Britain also launched a submarine search, helped by NATO allies.

Friday's passenger flight SK1755 from the Danish capital Copenhagen to Poznan in Poland was diverted by Swedish authorities before a collision occurred, the authorities said.

The flight was operated by Cimber, owned by Scandinavian airline SAS.

Russia's Defence Ministry denied on Sunday that its airplane had come close to colliding with a civilian airliner, official news agency TASS reported.

"A flight was carried out in strict accordance with international rules on air space and did not violate the borders of other countries and was at a safe distance from the flight paths of civilian airplanes," Defence Ministry spokesman General Major Igor Konashenko was quoted as saying.

Swedish military had said the Russian jet was flying with its transponder (a communications device which makes it easier for an airplane to be located) switched off, making it difficult for commercial traffic control to see it.

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"The military aircraft had no transponder but we discovered it on our radar and warned the civilian air traffic control in Malmo," Daniel Josefsson of the Swedish battle command centre, told daily newspaper Dagens Nyheter on Saturday.

"This is serious. This is inappropriate. It's outright dangerous when you turn off the transponder," Swedish Defence Minister Peter Hultqvist told Swedish radio on Saturday.

Earlier this month NATO complained Russian military aircraft are posing a threat to civilian planes by turning off communications devices and failing to file flight plans.

NATO warplanes have had to scramble 400 times this year in response to an increase in Russian air activity around Europe.