Specialist teams are examining the site of a US military helicopter crash in Norfolk, described as the size of a football pitch.

The investigation into last night’s Pave Hawk crash is being hampered by the fact that it was carrying munitions, meaning that bullets are scattered around the scene.

Emergency services worked throughout the night with the help of military personnel and volunteers after the aircraft from RAF Lakenheath came down at a nature reserve in Cley-next-the-Sea at around 6pm during a low-flying training exercise.

Speaking near the scene, Chief Superintendent Bob Scully of Norfolk Police said: “We have currently cordoned off about 400 square metres of the marshland area.

“The crash site itself I would describe as an area of debris on difficult terrain on the marsh that’s about the size of a football pitch.

“It’s not on the beach, although there are some bits of debris which are vulnerable to high tide.”

Mr Scully said it was too early to say what caused the crash.

“At the present time the coroner, who is responsible for the investigation into the deaths, is carrying out a daylight assessment of the situation and is then arranging for the deceased to be removed from the site,” he said.

He said the investigation would then be passed over to the Air Accidents Investigation Branch and its counterparts in the US.

Mr Scully told the press conference: “The crashed aircraft did contain ammunition.

“That ammunition is not of any great significance. It is bullets, if you will, but those are scattered about that area that I just described to you, and so the site is hazardous to members of the public and those people that would normally visit this area for birdwatching and other nature-interest activities, so for the present time we will be assisting and working with the military to ensure public safety by restricting access to that area.”

A second helicopter from RAF Lakenheath was also in the area at the time of the crash and set down on the marshes to try to assist.

Mr Scully refused to speculate on whether the second helicopter had any involvement in the incident but said that, as it was nearby at the time, it made sense that it went to help.

He went on: “We are moving from a potential recovery operation to one of preserving the scene and carrying out an investigation.

“As you would expect, we in the UK police have expressed our condolences to our US colleagues for the loss of some of their crew. It’s a desperately sad time. The US authorities have been responsible for notifying the next of kin of those who have sadly died.”

Residents told of hearing a “heavy and very unusual” sound overhead as the helicopter - which specialises in recovering troops from war zones - plummeted into marshland at the Norfolk Wildlife Trust Cley Marshes Nature Reserve.

Inquiries into the cause of the crash, as well as the recovery of the wreckage and the second aircraft, are expected to take a number of days to complete, due to the geography and the munitions from the crashed helicopter.