Four people are believed to have died in a helicopter crash in Norfolk, police said tonight.
The incident happened in Cley-next-the-Sea, in the north of the county.
Norfolk Police said: "Police are currently dealing with a single helicopter crash in the Cley area, on the North Norfolk coast. There are believed to be four fatalities. Officers are on the scene, with a 400 metre area cordoned off."
A spokesman for the Royal National Lifeboat Institution said: "We were asked for three lifeboats to respond to reports that an aircraft had possibly ditched in the sea.
"Lifeboats 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."
There were reports that a US military helicopter was involved. It was understood that there was no British military involvement.
With reports that the helicopter involved was a US military Black Hawk from RAF Lakenheath in Suffolk, aviation expert Chris Yates said the accident would "obviously have an impact" on Black Hawk operations.
Speaking on BBC News, Mr Yates said Black Hawks were primarily in use with the US military but that there were a number in the RAF fleet.
Mr Yates said: "The Black Hawk has a relatively good safety record and can fly in various different modes, so it's quite surprising that this has happened. The weather could have played a part.
"If something has gone wrong, if there has been some sort of defect thane obviously this will have an impact on Black Hawk operations."
The base is a Royal Air Force station near Mildenhall, Suffolk. I t 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."
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.
A number of cordons are in place, closing nearby residential streets.
Around a dozen emergency vehicles from the fire brigade, coastguard and police are at the scene.
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."
The 48th Air Wing of the US Air Force based at RAF Lakenheath tweeted: "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."
Pave Hawks are used for combat search and rescue, mainly to recover downed aircrew or other isolated personnel in theatres of war.
The 48th Fighter Wing, also known as the Liberty Wing, is assigned to the United States Air Forces in Europe (USAFE).
In addition to HH-60G Pave Hawks, it is home to squadrons of F-15 Eagle tactical fighter planes and F-15E Strike Eagle dual-role fighters.
Local reports said that residents had heard F-15 planes flying over the scene of the crash.
Helen Terry, 43, from Salthouse, near Cley, 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."
Norfolk Wildlife Trust said on their website they were "shocked" to hear of the crash.
A statement said the crash happened "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.
"It is likely the reserve will be closed for at least tomorrow, Wednesday 8th January, while the incident is investigated."
Peter and Sue McKnestiey, who run Cookies crab shop in Salthouse, have been making cups of tea for the search teams.
Mrs McKnestiey 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."
A spokesman for RAF Lakenheath said: "A US Air Force HH-60G Pave Hawk helicopter crashed at about 6pm today near Salthouse on the Norfolk coast.
"The aircraft, assigned to the 48th Fighter Wing, was on a low-level training mission when the crash occurred.
"The conditions of the four crew members remain unknown at this time.
"As soon as additional details become available they will be provided."
Superintendent Roger Wiltshire said: "The helicopter had a crew of four and sadly at this time we believe that all four crew members have died.
"We will shortly be making an assessment of the scene to make sure it is safe.
"We believe there is some ammunition on board the helicopter."
A 400m cordon is expected to stay in place around the scene for up to 24 hours.
He added that investigators will "do what they can tonight" but some tasks may need to wait until the morning.
The US Air Force will be involved in the investigation, Mr Wiltshire confirmed.
He added that the aircraft came down on the marsh.