Four Suffolk-based crew members died after a US military helicopter crashed in Norfolk on a training mission, it has been confirmed this morning.

The RAF Lakenheath-based Pave Hawk aircraft was taking part in a low-flying exercise when it came down in a nature reserve in Cley-next-the-Sea in the north of the county near Salthouse at around 6pm yesterday.

Emergency services have been working at the scene throughout the night with the help of military personnel and volunteers.

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

The aircraft was carrying munitions at the time of the crash and police said they were investigating the nature of the weaponry, which may pose a risk to the public.

A statement from the 48th Fighter Wing of RAF Lakenheath this morning said: “Four airmen were killed in a US Air Force HH-60G Pave Hawk helicopter crash at about 6pm yesterday near Salthouse on the Norfolk coast.

“Names of the airmen killed in the crash will be released 24 hours after next-of-kin notifications.

US military officials are coordinating the recovery efforts with the UK police and the Ministry of Defence.

“The authorities have secured the crash site and established a cordon.

“The aircraft, assigned to the 48th Fighter Wing, was performing a low-level training mission along the coast when the crash occurred.”

The HH-60G Pave Hawk helicopter was on a training mission from nearby RAF Lakenheath – a base for the US Air Force – which lies on the Suffolk/Norfolk border, around 50 miles from the crash site.

Details of the four crew members will not be released until next of kin have been informed and it is not believed that anyone in the surrounding area has been injured, police said.

It is thought emergency crews on scene this morning will wait until daylight to continue their investigation into the wreckage. It is expected to take a number of days. A 400m cordon remains around the crash site.

Police have held their first press conference as specialists examine the site described as “the size of a football pitch” with bullets “scattered” around.

Meanwhile eyewitnesses have revealed how they heard a “heavy and very unusual” noise moments before the crash.It has also emerged that the aircraft was from the biggest US base in England.

Sarah Hamlin, assistant chief constable at Norfolk Constabulary, said: “I would like to pass on my condolences to the family, friends and colleagues of those US Air Force personnel who have sadly lost their lives in this tragic incident.

“Emergency services, the military, partner agencies and volunteers have been working through the night to deal with this difficult situation on our coastline and I would also like to thank them for their professionalism and resilience.

“As our inquiry moves on today and the recovery of the aircraft begins, I would urge the public to stay away from the area – the cordon and road closures are in place to allow our experts to carry out these processes safely and there is no risk to members of the public if this section of marshland is avoided.”

Firefighters and police officers were sent to the scene at Norfolk Wildlife Trust’s Cley Marshes nature reserve and lifeboat crews were also initially called out to assist following reports of the crash in the rural village.

However, the lifeboats returned to their base on hearing the accident had happened just inland and not into the sea as first feared.

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, and remains at the scene while the investigation continues.

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.

A police spokesman said that an environmental assessment will also take place.

A 400m cordon protects the crash site and the A149 through Cley is closed.

Labour leader Ed Miliband said on Twitter: “My thoughts are with the friends and family of those who have died in the terrible helicopter crash in Norfolk.”

Matthew Barzun, US ambassador to the UK, tweeted: “My thoughts & prayers are with the four U.S. airmen tragically lost in Norfolk last night & their families.”

Bob Scully, chief superintendent at Norfolk Constabulary, added this morning: “In order to carry out a thorough investigation in a safe manner, the area will remain cordoned off with no access for the general public.

“We will be working with our partners at the Ministry of Defence, Air Accident Investigation Branch and US Air Force to gather all evidence from the scene and then recover the aircraft.

“This is difficult terrain with marshland and tides coupled with wreckage containing munitions covering a large area.

“We must undertake this investigation and recovery operation in a careful and methodical way so we can provide answers as to why this crash happened.

“For reasons of safety it is essential that members of the public adhere to the cordon. The popular activities of walking and bird-watching in this area will therefore be restricted until we have completed these tasks and ensured the marshes are safe.”

Residents were allowed to stay in their homes, although pedestrians and motorists were cleared from the scene.

A Norfolk Constabulary spokesman added: “A USAF Pave Hawk HH60 helicopter from RAF Lakenheath crashed at the north end of East Bank on the Norfolk Wildlife Trust Cley Marshes Nature Reserve at around 7pm yesterday.

“Police can now confirm that the four occupants in the helicopter sadly died in the crash. Details of the deceased will not be released until next of kin have been informed.

“It is not believed that anyone in the surrounding area has been injured.”

Norfolk Wildlife Trust said on their website they were “shocked” to hear of the crash.

A statement said the aircraft came down “on the shingle bank at NWT Cley Marshes nature reserve, and our immediate thoughts are for the families of those who sadly lost their lives.”

The statement added that the reserve will be closed for at least today while the incident is investigated.

The 400m cordon is expected to stay in place around the scene for up to 24 hours.

The Norfolk Constabulary spokesman this morning added: “Investigators from the police and other partners have visited the crash site to assess the scene.

“Due to the geography and the munitions from the crashed helicopter, inquiries into the cause of the collision, the recovery of the wreckage and second aircraft and an environmental assessment are expected to take a number of days to complete.

“To ensure the safety of people in the area and those involved in the ongoing inquiries, members of the public are asked to respect the cordons that are in place.”

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.

Peter and Sue McKnespiey, who run Cookies crab shop in Salthouse, live near the crash site.

Mrs McKnespiey said: “We were watching TV at about 7pm. We heard the helicopter come over very fast and very low.

“I don’t know about engines but I am used to the sound of helicopters and this sounded very heavy and very unusual.

“My gut instinct was there was something wrong. We’ve now heard four people have died and it’s just awful. I keep hoping the helicopter I heard isn’t the one that crashed. I think the whole village will be devastated when it realises what’s happened.”

Cley is 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.

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.”

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.”

Helen Terry, 43, from Salthouse, said: “We heard the helicopter fly over. We assumed it was just heading out to sea for training exercises. It’s a daily occurrence and we’re quite used to it.

“We live less than half a mile from where it’s happened and we didn’t hear any bang. The first we heard was when we saw emergency crews rushing to the area.

“It’s something locals are used to and we’ve never had any safety concerns.”

A resident who did not want to be named said: “We heard the helicopter fly over. There wasn’t any bang but soon after we heard some jets fly over very low. It was obvious it was part of a search operation and it shook our house.”