With fighter jets roaring overhead, NATO has inaugurated a military center in the Lithuanian capital amid growing regional concerns over Russia's military presence, sparked by the fighting in Ukraine.

Lithuania, which like its Baltic neighbors Latvia and Estonia, was occupied for nearly five decades by the Soviet Union, has published a manual on what to do if war breaks out and volunteers are signing up for the Home Guard in record numbers.

NATO Secretary-General Jens Stoltenberg joined Lithuanian President Dalia Grybauskaite on Thursday to open the alliances' force integration unit, one of six in an eastern expansion of the partnership.

The alliance says that Moscow has increased its military air activity near the Baltic countries' borders with a growing number of intercepts conducted by NATO fighter jets.