NATO's chief says Russia's increased military activity in the skies above the Baltic region is "unjustified" and poses a risk to civil aviation because aircraft regularly fail to file flight plans or communicate with air controllers.

Jens Stoltenberg says alliance fighters have intercepted planes more than 100 times in the region so far this year, a threefold increase on 2013.

During a visit to an air base in NATO-member Estonia on Thursday, Stoltenberg said the alliance "should stay vigilant" because of Russia's increased "military air activity," even though most flights occurred in international airspace.

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