Four American Marines were killed when their Osprey aircraft crashed in the frozen water of the Arctic Circle during a NATO exercise, authorities said Saturday.

While information is still coming in, initial reports indicate the accident was unrelated to the conflict in Ukraine

“It is with great sadness we have recived the message that four American soldiers died in a plane crash last night,” the Norwegian prime minister tweeted. “Our deepest sympathies go to the soldiers’ families, relatives and fellow soldiers in their unit.”  

The Marines were working on an exercise called Cold Response, which has the Americans working on flights, survival, and combat in the coldest climate on our planet. The names of the soldiers have not yet been released, as the U.S. Defense Department hasn’t notified all relatives impacted by the tragedy.

Norwegian representatives said a search and rescue mission was launched immediately, but the inhospitable climate of the frozen waters were too much to overcome for even the best trained soldiers.

Starting on March 14, around 30,000 troops, 220 aircraft, and 50 vessels from NATO countries assembled just west of Russia for drills unrelated to the conflict in Ukraine.

The Osprey model has been involved in a few deadly crashes in recent years, with 3 marines killed in 2017 in a similar crash off the coast of Australia.