A US Air Force B-1B Lancer bomber crashed on Monday in the northern state of Montana but its four-strong crew managed to eject before impact, the military said.

The plane had taken off from Ellsworth Air Force base, home of the 28th Bomb Wing, on what was described as a routine training mission.

The cause of the crash was not immediately known and all four crew members suffered "some injuries" during the incident, according to a statement.

"We are actively working to ensure the safety of the crew members and have sent first responders to secure the scene and work closely with local authorities at the crash site," said Colonel Kevin Kennedy.

"Right now all of our thoughts and prayers are with the crews and their families," he said.

The B-1B is an updated version of the Rockwell corporation's long range B-1 bomber, which was originally designed to carry nuclear weapons.