All four people on board a United States military aircraft that crashed in northern Norway on Friday have died during a NATO exercise, local police said Saturday.

The MV-22B Osprey aircraft belonging to the U.S. Marine Corps was taking part in a military exercise called "Cold Response" when it crashed in a remote region.

"As far as the police are aware, all four are of American nationality," police said. Prime Minister Jonas Gahr Stoere tweeted condolences over what he said was the death of four Americans.

Rescue services reached the crash site by land early on Saturday after helicopters were unable to land due to poor weather conditions. Gale-force winds were blowing, heavy rains were falling, and there was a risk of avalanches, according to local weather forecasts.

"Police reached the crash site at around 1:30 a.m. CET (12:30 a.m. GMT). It is regrettably confirmed that all four onboard the plane have perished," Ivar Bo Nilsson, head of the operation for Nordland police, said in a statement.

Police were investigating the cause of the crash although their work was halted because of the weather conditions. The work was set to resume once the weather improves.

Some 30,000 troops from 27 countries are involved in Cold Response, an exercise designed to prepare NATO member countries for the defense of Norway.