Four people were killed when USAF Pave-Hawk helicopter crashed on a routine low fly training mission on the north Norfolk coast, police have said.
The helicopter is an HH-60G Pave Hawk, based at RAF Lakenheath in Suffolk, a spokesman for the USAF confirmed.
The helicopter crashed in a bird sanctuary raising suggestions that it could have been brought down by a bird strike – a regular hazard for aircraft.
Emergency services are at the scene in the Cley area with a 1,200ft (400m) area cordoned off.
Police have said residents can stay in their homes but pedestrians and motorists are being diverted away as there is live ammunition on board, which could pose a threat to the public.
RAF Lakenheath said: “We can confirm that one of our HH-60G Pave Hawk helicopters was involved in an incident during a training mission outside Cley-Next-The-Sea.”
Witness Michael Girling said: “I saw it go down. At first I thought it had landed on the beach but then realised it had ditched in the marsh.
“I actually saw it go down but I did not see it ditch. It went down in the nature reserve. There are flashing lights everywhere. No one is allowed anywhere near the area.”
Another witness Brian Egan said he saw two military choppers carrying out low flying manoeuvres. They were flying extremely low considering it was night. They do exercises around here.
“Later I heard there had been an accident and a helicopter had ditched.”
Pave-Hawk helicopters are used by the US military for special operations in war zones. They are not heavily armed but are equipped with rapid fire machine guns and are used in special operations.
There was speculation on Tuesday night that a bird strike could have been responsible for bringing the aircraft down as the aircraft crashed in a nature reserve. Paul Beaver, a pilot, told Sky news: 'It's a wonderful place for waterfowl it could have been a bird strike, night time birds do fly there.'
The role of HH-60G Pave Hawk helicopters is to conduct rescue operations in hostile environments to recover isolated soldiers during wartime.
They are also used for civilian search and rescue missions, medical evacuations, disaster responses and humanitarian assistance.
The Pave Hawk is a modified version of the Army Black Hawk helicopter and has upgraded communications and navigation technology making it ideal for rescue missions.
They are designed for night-time low-level operations and have an automatic flight control system, night vision goggles, lighting and an infrared system. The Pave Hawk was used after Hurricane Katrina in September 2005 and in the aftermath of earthquake and tsunami in Japan.
Prince Harry qualified as an Apache Aircraft Commander at Wattisham Airfield, based in Suffolk. He carried out months of training at the base ahead of his assessment in July last year.
Emergency services have cordoned off a 400 metre area around the crash site on the north Norfolk coast.
Six appliances from Norfolk Fire Service are currently in attendance, including four pumps, a water carrier and an environmental protection unit.
A spokesman said the first unit arrived at the scene at 19:53 GMT.
It was initially thought the aircraft had ditched in the North Sea.
A spokesman for the Royal National Lifeboat Institution said: "We were asked for three lifeboats to respond.
"Crews from Wells, Sheringham and Cromer were launched at the request of the coastguard but were stood down when it was confirmed that the aircraft had come down over land."
The base is a Royal Air Force station near Mildenhall, Suffolk. It hosts US Air Force units and personnel.
Cley is a picturesque village, one mile east of Blakeney and four miles north of Holt, on the main coast road between Wells and Sheringham.
Well known for its windmill and church, the village has a nature reserve famous as a birdwatching site.
The area is popular with walkers and tourists, who enjoy the views and wildlife.
Richard Kelham, chairman of Cley Parish Council, said: "It looks as though the military helicopter has come down in the middle of the bird reserve. The incident is very sad and there is a 400m cordon surrounding the area."
Locals reported hearing helicopters performing manoeuvres overhead late at night over the last few days.
Police said they had arrived on the scene around 8pm, after being called by the fire service.
The RNLI said three of its boats were called out at about 7.45pm but were called back
Some residents have reported hearing F-15 aircraft searching overhead.
Cley Marshes is cordoned off and the coastal road is also closed.
Cley artist Rachel Lockwood, from the village’s Pinkfoot Gallery, said : “We had never seen so many police cars and fire engines so went to have a look.
“The beach road to Cley is sealed off. There are lots of fire engines near the Dun Cow pub at Salthouse. A helicopter is hovering over the marsh with a light beaming down.
“Someone said it was a helicopter down, and a coastguard told us to clear the area, saying something about ammunition.
“A guy at Salthouse said he had heard a lot of helicopter manoeuvres at night recently.
“If it is down on the marsh it might be difficult to find because of the flooding after the sea surge.
“There are police cars at Cley and Salthouse car parks, but you cannot see any fire or wreckage.”