Four American servicemen killed in a helicopter crash during a training exercise on the north Norfolk coast in England have been officially identified.

Captains Christopher S. Stover and Sean M. Ruane, Technical Sergeant Dale E Mathews and Staff Sergeant Afton M Ponce were killed when their Pave Hawk helicopter came down on a marsh near Cley-next-the-Sea last night.

RAF Lakenheath, where US Airforce 48th Fighter Wing is based, said that Captains Stover and Ruane were the pilots, while Tech Sgt Mathews and SSgt Ponce were acting as special mission aviators.

Colonel Kyle Robinson, 48th Fighter Wing commander, said: "We continue to think of the loved ones who are experiencing such a tragic, sudden loss.

"The Liberty Wing feels as though it has lost members of its family, and we stand by to support one another and these airmen's families during this difficult time."

Authorities have said that the bodies of those killed in the crash are unlikely to be recovered until tomorrow.

Air accident, RAF and US investigators have spent the day at the scene of the tragedy, where debris is believed to have been strewn across an area the size of a football pitch.

A statement from 48th Fighter Wing said that the helicopter was performing a low-level training mission along the coast when the crash took place.

The investigation is being hampered because it was carrying munitions, meaning that bullets are scattered around the scene, investigators said.

Norfolk Police confirmed paramedics were not needed to treat those inside the helicopter, suggesting they died on impact or soon after.

A 400-metre police cordon is expected to remain in place at the scene until Monday and the public have been asked to stay away.

Investigators are aware of how much ammunition was on board and are attempting to account for it all.

Military investigators from the US and UK are expected to work together to establish exactly what happened once the police involvement is over.

Speaking in nearby Salthouse this afternoon, chief superintendent Bob Scully said: "You would be very much mistaken if you thought this would be a quick process.

"It all hinges on our ability to establish what happened and the removal of the casualties, who are sadly deceased, could disrupt the evidence so this must be done methodically step by step."

A derivative of the more famous Black Hawk helicopter, the Pave Hawk gets its name from the Pave acronym standing for Precision Avionics Vectoring Equipment.

They are used for combat search and rescue, mainly to recover downed aircrew or other isolated personnel in theatres of war.

They have a four-man crew and can carry up to 12 troops.

Typically, training flights would replicate as closely as possible real missions which would mean weapons and ammunition would be carried.

Meanwhile, at least one person was killed this afternoon when a US Navy helicopter carrying five crew members ditched in the Atlantic off the Virginia coast near Norfolk.

Three people have been evacuated to a civilian hospital after the MH-53E Sea Dragon aircraft crashed and a search continues for the fifth crew member.