Sweden is poised to join NATO, possibly within days, after gaining the final outstanding ratification last week.

The exercises - involving over 20,000 troops from more than a dozen countries - are taking place in a high north being transformed by the accession of Sweden and Finland, who turned their backs on long histories of non-alignment after Russia invaded Ukraine two years ago.

In Bodoe in northern Norway, U.S. and Norwegian F-35s joined Finnish F-18s and Swedish Gripen fighter jets flying low across the sky above Camp Bodin military base, part of a first test of a joint Nordic air operations centre, a key part of the drills.

"The new concept now is that we try to integrate even further and that is possible because of the new NATO membership of Finland and the Swedish NATO membership shortly to come," said Major General Rolf Folland, Norwegian Air Force Chief and head of the new centre.

"Then the last hindrance for total integration is taken away and that makes it possible for us to be able to work as one air power in peacetime, crisis and war."

More than 300 officers and specialists are working to coordinate and control the aircraft taking part in the exercises, about half of them temporarily operating from Camp Bodin.

The drills in the far north of Norway, Sweden and Finland are part of the alliance's overarching Steadfast Defender 2024 exercises rehearsing how U.S. troops could reinforce European allies on NATO's eastern flank and put new regional defence plans to the test.

About 90,000 troops are due to join the wider NATO exercises running through May that include more than 50 ships as well as planes, tanks and other armoured vehicles.

NORDIC FLEET

Sweden, Norway, Finland and Denmark agreed last year to begin creating a unified Nordic air defence aimed at countering the rising threat from Russia.

Integrating the Nordic air forces will create a regional fleet of around 250 fighters, on par with that of a military power such as Britain, offering credible deterrence against would-be attackers.

"This is going to be a massive contribution in terms of air power and control of the skies over the entire Baltic, Nordic, high north region," said William Alberque at International Institute for Strategic Studies, a leading research center.

Sweden, which has not been at war in two centuries, has edged ever closer to NATO in recent decades - in part because its own military was scaled back after the Soviet Union's collapse - taking part in missions in places such as Libya.

"We have a lot of strengths when it comes to operating and connecting our systems together and we can bring that to the Nordic nations and to NATO," said Major General Jonas Wikman, chief of the Swedish Air Force.

Among Sweden's strengths are a fleet of Gripens, a rugged fighter that can be readied for new missions minutes after landing and operate from small temporary air strips, a capability generating interest among allies.

"Right now, both NATO and the U.S. are looking quite a bit at Sweden and Finland," Colonel Peter Greberg, commander of the Swedish Air Force's 21st wing, said on the sidelines of a demonstration of the jet in Lulea, in northern Sweden.

"I had a NATO general here last week who visited us precisely to look at this concept."

Sweden's armed forces may well face the prospect of joining NATO before the current exercises are actually over. "I have a bottle of champagne ready to pop whenever the membership is granted," Norwegian Air Force chief Folland said.

(Reporting by Niklas Pollard; Editing by Stephen Coates)