The Swedish Air Force on Tuesday, March 24, tracked four Russian combat aircraft flying with their transponders turned off over the Baltic Sea, officials said.

The Russian planes — two long-range, nuclear-capable Tu-22M3 bombers and two Sukhoi Su-27 fighters — were flying in international airspace, according to Sweden's Armed Forces.

Swedish Foreign Minister Margot Wallstrom called it "unacceptable" for the Russian planes to be flying with shut-off transponders that are necessary for identifying aircraft on radar, calling it violation of international aviation rules.

"This has happened now on a number of occasions and in a very challenging way," Wallstrom told reporters in Stockholm. "We are tired of always having to protest against this kind of ... breach of rules."

NATO and Sweden, which is not a member of the alliance, have reported an increase in Russian air maneuvers over the Baltic Sea in recent years.

Tuesday's sighting comes as Finnish and Swedish military aircraft begin training with U.S. fighters over the Baltic Sea, and American and NATO forces continue military exercises in the Baltic countries.

The Swedish military said that the threat against Sweden had not grown but that the armed forces were watching "increased activity" in the region.