Four American soldiers have been killed in an aircraft crash in Norway, the country’s Prime Minister Jonas Gahr Støre announced on Saturday, hours after the crash took place.

“It is with great sadness we have [received] the message that four American soldiers died in a plane crash last night,” Støre wrote on his Twitter account on Saturday.

“The soldiers participated in the NATO exercise Cold Response. Our deepest sympathies go to the soldiers’ families, relatives and fellow soldiers in their unit,” the 61-year-old added.

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The incident had taken place around 6pm local time on Friday when a V-22B Osprey with a crew of four American soldiers headed to Bodoe crashed near Graetaedalen in Beiarn.

At the time, the US Marine Corps had tweeted, “We can confirm an incident has occurred involving a Marine Corps MV-22B Osprey aircraft. The aircraft was conducting training in Norway as part of Exercise COLD RESPONSE 22 at the time of the incident.”

However, as of now, it not clear what caused the military transport aircraft to crash: “The cause of the incident is under investigation, and additional details will be provided as available,” the USMC said in a tweet.

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Exercise Cold Response is an annual NATO exercise that takes place in Norway and is not related to the ongoing war between Russia and Ukraine.

This year, as many as 30,000 troops, 220 aircraft and 50 vessels from 27 different countries participated in the military exercise, with non-NATO members such as Finland and Sweden also participating.

The exercise began on March 14 and is slated to conclude on April 1, and will carry on as per schedule despite the tragic deaths of the four American soldiers.