Russian Tu-22M Back Fire Bomber. Wikipedia Swedish fighters intercepted four Russian planes flying in international air space over the Baltic sea with their transponders turned off, the country's military said on Tuesday.

Relations between the West and Russia have soured as a result of Moscow's role in the conflict in Ukraine and its annexation of Crimea, while increased Russian military activity in the Baltic has added to tension.

Sweden, which is not a member of NATO, said it had intercepted two Russian Backfire bombers which were being escorted by two Flanker fighter jets.

"The threat level to Sweden has not increased, but the Swedish armed forces track, as always, ... the increased activity in our neighborhood," the country's military said in a statement.

Russia has stepped up flights near Europe's borders over the last couple of years forcing jets from NATO countries and allies like Sweden to scramble hundreds of times.

In December, Sweden and Denmark summoned Russia's ambassadors to the two countries to complain about an incident in which a Russian military jet flew with its transponder - a communications device that makes it easier for an airplane to be located - off and nearly collided with a commercial jet.

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

In a further sign of tension, Russia's ambassador in Denmark this weekend that said Moscow could aim nuclear missiles at Danish warships if Denmark joins NATO's missile defense system.

(Reporting by Simon Johnson Editing by Jeremy Gaunt)