STOCKHOLM (AP) — Sweden’s military says it has observed four Russian military planes in the Baltic Sea region, including two bombers, flying with shut-off transponders that are necessary for identifying aircraft on radar.

The Swedish Armed Forces says it trailed the Russian planes on Tuesday in international skies. The planes included two long-range, nuclear-capable Tu-22M3 bombers and two Sukhoi Su-27 fighters.

The military said that the threat against Sweden had not grown but that the armed forces watching “increased activity” in the region. Sweden and NATO have reported several air violations by Russian military planes in the region during the past year.

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.