Four people were killed when a US Air Force helicopter crashed in a coastal area of eastern England during a training mission.

The helicopter, a Pave Hawk variant of the Black Hawk, crashed at about 6pm local time near Salthouse on the coast of the country of Norfolk, the USAF said. All those on board were killed.

Investigations are under way in the remote marshland area in eastern England to find out the reason behind its crash. Norfolk police chief superintendent Bob Scully says it's a slow process.

"A lot hinges on our ability to understand what happened to the aircraft that crashed. And that includes the detailed investigation that needs to be done," Scully said. "Removal of casualties, sadly deceased from the aircraft, under those circumstances, can disrupt the evidence. It has to be done methodically, step by step. So it will take some time to move the deceased from the aircraft, possibly until tomorrow."

The aircraft was based at the Royal Air Force base in Lakenheath, in neighbouring Suffolk, which hosts USAF units and personnel.

The helicopter, assigned to the 48th Fighter Wing, was flying low at the time of the crash, the USAF added.

Police in Norfolk cordoned off the area around the crash site, and several emergency vehicles were at the scene.

Pave Hawks are often used for combat search and rescue missions, mainly to recover downed air crew members or other personnel during war.

They typically practice flying low and fast, often at altitudes of hundreds, rather than thousands, of feet.

Pave Hawks have been deployed in numerous overseas missions, including to Japan in the wake of the tsunami in 2011 and to support operations in Afghanistan, Iraq and Libya.