Four US soldiers have been killed when two helicopters crashed on a training exercise at Joint Base Lewis-McChord, near Tacoma, reports quoting a military spokesman said.

Lt. Col. Gary Dangerfield of Joint Base Lewis-McChord, told the Seattle Times the circumstances of Monday night’s crash remained unclear.

“Our thoughts and prayers go out to the families of the deceased,” Lt. Col. Gary Dangerfield said, promising a “thorough investigation”.

Joint Base Lewis-McChord is a joint US army and air force facility.

The facility supports more than 100,000 military and civilian personnel, contract employees and retirees, the Seattle Times reports.

Lt. Col. Gary Dangerfield said two OH-58 Kiowa choppers had crashed in the aviation training area south-west of the base at around 20:00 local time in clear skies.

“We will conduct a thorough investigation,” he said.

“We will do everything in our power to support the families of the brave soldiers who died this evening.”