Four crew members have been killed after a USAF Pave-Hawk crashed in north Norfolk during a routine training exercise, near the Royal Air Force base in the east coast of England.
The aircraft was taking part in a low-flying manoeuvre and was carrying live ammunition when it was brought down.
The incident took place at about 6 pm on Tuesday (7 January).
The HH-60G Pave Hawk helicopter, based at RAF Lakenheath in Suffolk, usually carries four crew members on board, said officials.
"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. The aircraft, assigned to the 48th Fighter Wing, was on a low-level training mission when the crash occurred," said an RAF Lakenheath spokesperson.
A Pentagon official in the US later said all crew members were presumed dead. The details of the victims will be released after their relatives have been notified.
As the aircraft went down in a bird sanctuary, there are suggestions it could have been brought down by a bird strike. Following the crash, a 400-metre cordon was set up by the authorities around the site in the nature reserve run by Norfolk Wildlife Trust.
The cordon is likely to be in effect for at least 24 hours.
Emergency officials from the fire brigade, coastguard and police are at the scene probing the incident.
"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," said the Royal National Lifeboat Institution (RNLI).
Sue McKnespiey, who lives near the crash site, told the Guardian: "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."
The 64ft-long Pave-Hawk aircraft are used in anti-terror operations across the world and for rescue missions in hostile zones.