NATO scrambled fighter jets twice in two days to intercept Russian military aircraft over the Baltic Sea, it said Tuesday amid reports that Russian military activity in the region is increasing.

Lt. Col. Robert Gericke said the Russian aircraft were flying in international airspace and had not violated the territory of alliance members.

Two Canadian F-18 Hornet jets were scrambled from the Siauliai Air Base in Lithuania on Monday to intercept a Russian Ilyushin-20 surveillance aircraft, which they shadowed for some 15 minutes, NATO said.

"Once identification was successful, the intercept mission was completed and the two Hornets returned to their base," a NATO statement said.

Earlier, the Latvian military tweeted that NATO F-16 jets were dispatched on Tuesday to intercept a Russian Ilyushin-20 surveillance aircraft over the Baltic Sea. Gericke confirmed that NATO jets had also intercepted a Russian aircraft that day, but could not immediately provide more details.

NATO, which has 16 fighter jets in the region monitoring Baltic airspace, said it regularly launches jets to identify "unknown or potentially hostile aircraft" in the proximity of national airspace.

There were two similar incidents in the region on Oct. 7 and Sept. 11, but on neither occasion did the Russian aircraft constitute a threat to NATO forces, the alliance said.

In the past five days, the Swedish Navy has been combing the Stockholm archipelago for signs of a foreign submarine that officials suspect entered its territorial waters illegally. It hasn't officially linked Russia to the suspected intrusion.

The Finnish military says that Russian military aircraft have violated the small Nordic country's airspace five times this year, and the Environment Institute said Russian military ships had twice intercepted one of its research vessels in international waters.

On Sept. 5 an Estonian security service officer was detained on the Russian border — Estonia and Russia disagree on which side of it — and is still in custody in Moscow.