NATO jets were scrambled Tuesday to escort Russian fighters and nuclear-capable bombers flying near the Baltic states and Sweden with their transponders switched off, sparking protests over the danger they posed to civil aviation.

Lithuania's defence ministry spokeswoman Asta Galdikaite said NATO air policing aircraft identified two Tu-22 type bombers and two SU-27 jets. The Swedish military also confirmed the aircraft showed up on their ground radar.

"The flights conducted with switched-off on-board transponders are among other things a risk to civil aviation as such flights are not visible on civil air traffic control radars," she told AFP.

"We are tired of having to repeatedly protest against these violations," Foreign Minister Margot Wallstroem told the Swedish TT news agency on Tuesday.

"We need to obtain Russia's respect for existing regulations and put an end to this, which has been incredibly challenging and even dangerous for civil aviation as well," she added.

In December a Russian military plane came dangerously close to a passenger plane from Cimber Air, of the SAS group, which had taken off from Copenhagen headed for Poznan in Poland, according to the Swedish military.

On Tuesday, the aircraft flew from mainland Russia to Kaliningrad, a Russian enclave on the Baltic Sea, sandwiched between Poland and Lithuania. Both are EU and NATO members, while Sweden is not.

Galdikaite said Lithuania had seen "an increase in the activity of military aircraft of the Russian Federation over the Baltic Sea this year", adding it was the first time since December that Russian bombers were spotted in the area.

Russia's seizure of Crimea, support for separatists in eastern Ukraine and stepped-up military drills have caused unease in the Baltic states and Poland, which lay behind the Iron Curtain a quarter of a century ago.

Lithuania's President Dalia Grybauskaite said last week that Russia sent nuclear-capable Iskander missiles to Kaliningrad, which could "reach even Berlin" .

US forces deployed a battery of Patriot surface-to-air missiles near the Polish capital Warsaw this weekend as part of ongoing drills.

NATO is also countering Moscow's moves by deploying more aircraft, ships and personnel for exercises on its eastern flank.

The 28-member alliance already runs an air policing mission in formerly Soviet-ruled Baltic states bordering Russia.

NATO has created a spearhead force of 5,000 troops capable of rapid deployment and will establish command centres in the Baltic states, Bulgaria, Poland and Romania by 2016.

US troops on exercises in eastern Europe are, meanwhile, keeping a high public profile.

Infantry soldiers in a convoy of Stryker armoured personnel carriers returning to Germany from exercises in the Baltic states via Poland and the Czech Republic are meeting and greeting civilians on their 1,800 kilometre (1,120 miles) journey dubbed the "Dragoon ride".