By Anne Kauranen

ROVAJARVI, Finland NATO countries are in the middle of Arctic military manoeuvres, vowing on Tuesday to defend their newest member, Finland, which is hosting its first joint NATO exercise since becoming the 31st member of the Western alliance in April.

The addition of Finland doubles the length of the border NATO shares with Russia, which launched a large-scale invasion of another neighbour, Ukraine, in February last year.

Nearly 1,000 allied forces from the United States, Britain and Norway, and also from neighbouring Sweden, joined some 6,500 Finnish troops and some 1,000 vehicles for the Northern Forest exercise, Finland's biggest modern-time land force drill above the Arctic Circle.

Russia's invasion of Ukraine convinced Sweden and Finland last year to ditch long-held policies of military non-alignment and seek security of NATO's collective defence commitment.

Finland formally joined on April 4, drawing a threat from Moscow of "counter-measures". Sweden hopes to be a member by the time of the alliance's summit in Vilnius in July.

Overseeing the exercise just a two-hour drive from the Russian border at one of Europe's largest artillery training grounds in Rovajarvi, northern Finland, Major General Gregory Anderson from the 10th Mountain Division of the U.S. Army said his country stood ready to defend Finland.

"We are here, we are committed. The U.S. Army is here training with our newest NATO ally to build that capability, to help defend Finland if anything happened," Anderson said.

His troops practised on the ground with their Multiple Launch Rocket Systems shipped from Germany, while their Finnish counterparts fired their Leopard 2A6 tanks, the Swedes and the Norwegians fired from CV90 infantry fighting vehicles and the UK troops deployed Warrior tanks.

Sweden remains Finland's closest military partner and is also both politically and tactically ready to defend its Nordic neighbour, the commander of the Swedish land forces said.

"It's of no question at all. We have prepared plans on what to do if we are to be part of the defence of Finland," Major General Karl Engelbrektson said.

Ilmari Laukkanen, a 20-year-old Finnish conscript who operated a field gun in the exercise but in his civilian life works at a family farm near the Russian border, said he would be ready to fight for Finland's new allies if needs be.

"Of course I would. If we are given something good then we give in return," Laukkanen told Reuters.

Thea Rimmereid, 21, from Norway's northern Porsanger Battalion, when asked if she felt ready to defend Finland said she had only been in the military for 13 months.

"But we will do the best if we need to," Rimmereid said.

Exercise leader Colonel Janne Makitalo said NATO allies should be cautious about drawing quick conclusions on their military procurement needs from the footage they see from Ukraine.

"Russia is currently able to destroy, on a monthly basis, about 10,000 Ukrainian UAVs (uncrewed aerial vehicles)," Makitalo said.

Expensive technological solutions might look good on YouTube, he said, but they can be easily detectable in a real war situation.

The exercises run from May 27 to June 2.

(Reporting by Anne Kauranen; Editing by Nick Macfie)