Colonel Sebastien Vallette swings back in his desk chair, minutes after landing his Mirage fighter on a Soviet-era runway.

A detailed map on his wall shows Lithuania, Latvia and Estonia, flanked by bold warnings: "Do Not Enter Russian Airspace".

For six years, NATO allies have taken turns patrolling the airspace of the three Baltic states, minnows which won freedom from the crumbling Soviet Union in 1991 and have testy ties with Moscow.

Vallette and six other French flyers under his command are the current eyes in the sky.

"The point about airspace sovereignty is to know who's doing what in it," Vallette told AFP at Zokniai, a huge, mostly-mothballed facility in northern Lithuania where 20,000 Red Army troops were once based.

"It's a combination of deterrence, protection, prevention and assistance," he added.

Vallette acknowledged Baltic history is "complicated", but rejected suggestions his job is about warning the resurgent Kremlin, which only withdrew its forces in 1994.

"Everyone wants to put Russia in focus, but there isn't a special focus," he insisted. "All countries conduct surveillance in their national airspace. There isn't a concept of a particular threat."

The Baltic trio joined NATO on March 29, 2004. The air patrols began immediately.

Most members of the 28-nation, trans-Atlantic alliance cover their own skies. But Lithuania, Latvia and Estonia are small and cash-strapped.

"It's not a good idea to have a void airspace above NATO territory. It's a matter of principle. Wherever it is. Voids are always a temptation," said Estonian Defence Minister Jaak Aaviksoo.

The trio's total population is 6.8 million, and their professional military, 20,500. They have 580 troops in Afghanistan, winning praise from NATO chief Anders Fogh Rasmussen for "punching above their weight".

Their air forces total 1,550 personnel and 35 aircraft, mostly helicopters and transporters.

"There's also a symbolic value in addition to the purely technical control of air borders," said Aaviksoo. "This is an indication of solidarity".

His Lithuanian counterpart Rasa Jukneviciene underscored that: "It's about 'NATO visibility'".

The Baltic states together pay 100,000 euros (133,000 dollars) a month, or 10 percent of operation costs. The patrolling state meets the rest.

The patrol mandate runs until 2014 but the trio are trying to get it extended.

"The indications about 2018 are pretty positive," said Latvian Defence Minister Imants Liegis. NATO member Norway is helping draft a post-2018 plan, which could involve creating a small joint Baltic air force.

"The air mission is the only allied footprint on the territory of the Baltic states," noted Lithuanian defence analyst Aleksandras Matonis. "It has no military meaning. But it has political substance".

The French served a first four-month stint in 2007 and are due back in 2011. Belgium, Britain, the Czech Republic, Denmark, France, Germany, the Netherlands, Norway, Poland, Portugal, Romania, Spain, Turkey and the United States have also taken part.

Vallette's 140-strong contingent -- it includes ground crew -- took over from the Germans on January 4 and gives way to the Poles on May 1.

Two fighters make joint routine sorties twice a day, lasting an hour and a half and mostly involving simple observation.

But the fighters can scramble any time to make visual contact with planes whose communications have gone offline, and may escort them out.

Russian aircraft bound for Moscow's Kaliningrad territory, west of Lithuania, have strayed into the Baltic states' skies without permission. The line between navigational error and "buzzing" can be fine.

"There's a kind of mutual understanding that means there isn't an escalation," said Vallette.