Royal Marines train US forces in Arctic over fears of Russian aggression
British Royal Marines are training their American counterparts in Arctic warfare for the first time.
Around 200 US Marines are taking part in the fighting and survival training in Norway.
It comes as the defence secretary tells Newsbeat there are concerns about "Russian aggression" in the region.
The training is happening ahead of a large Nato exercise called Cold Response, involving 15,000 troops from 14 countries.
The training is taking place in Porsangmoen, less than 200 miles from Norway's border with Russia.
Tension has increased recently between Russia and Western countries including the UK and US.
Russia's president Vladimir Putin has reportedly been moving troops into the north of the country.
Speaking to Newsbeat, Defence Secretary Michael Fallon said Russia had been acting aggressively.
"We have to be ready to deal with that kind of aggression and make sure that Russia knows that if it crosses the line we're ready to respond."
What is Nato?
The North Atlantic Treaty Organisation (or Nato) is the world's most powerful military alliance.
There are 28 member countries including the UK, USA, France and Italy.
It was set up after World War Two to stop the spread of communism by the old Soviet Union in Europe.
Nato says it promotes "stability and well-being in the North Atlantic area" and that an attack against one member will be considered an attack against all.
The new Arctic training for US marines is happening in an area the size of Belgium.
Royal Marines are teaching them how to build shelters, survive in sub-zero temperatures and move around efficiently in snow.
The Americans are mostly in their teens and early 20s and have to sleep rough in temperatures as low as -30C.
"The reason the Royal Marines come here is we know if we can operate here we can operate anywhere around the world," explains Major Adam Rutherford who is in charge of the training programme.
"So as a testing ground you couldn't get any tougher or more demanding."
In his interview with Newsbeat, the defence secretary explained a number of countries were caught out when the Russians took over the Crimea from Ukraine.
Since then, attention has turned to the Arctic Circle over fears the Russians will do something similar in that region.
"Because of global warming, the route there is beginning to open up," said Mr Fallon.
"It could become a sea lane for commercial traffic so it is important we keep an eye on what is happening in the high north."
Mr Fallon denied relations with Russia are as bad as the days of the Cold War, a period of high tension between western countries and the old Soviet Union in the last century.
"We're not back to those days yet. We'd hoped Russia would have been more of a partner with us.
"Russia's shown by what it did in the Crimea and the pressure it's putting on the Baltic states that it wants to compete against Nato.
"We don't want that, and we have to be ready to respond to that."
While it's been a tough three weeks for the US Marines in Porsangmoen, having to use their new combat skills against the Russians will be a very different prospect.
But the defence secretary is confident this type of training will send a strong enough message to the Russians.
"They watch very carefully at what we're doing and were watching what they're doing and we need to do that."
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