The US Osprey aircraft "was reported missing at 18:26 (1726 GMT) south of Bodo" in northern Norway in bad weather, the regional emergency services (HRS) said in a statement.

The four-person crew were taking part in the Cold Response military exercises involving 30,000 people from NATO and partner countries.

Rescuers searching from the air later saw signs of the aircraft in the area where it went missing but the weather was too bad for them to land, the statement said.

Rescue teams and police are heading to the area, it added.

"We are not at the site itself so we know nothing of the four people who were on board. But we know it is a crash site," HRS spokesman Jan Eskil Severinsen said on the NRK television channel.

Cold Response 2022 aims to test how Norway would manage allied reinforcements on its soil in the event that NATO's mutual defence clause were triggered.

This week's exercises came amid high tension between Russia and NATO over Moscow's invasion of Ukraine, but they were planned long before that offensive began on February 24.