File photo of a USAF HH-60G Pave Hawk helicopter.
A US military helicopter crashed while on a low-level training exercise in eastern England on Tuesday, killing all four crew members, military officials announced.
"A US Air Force HH-60G Pave Hawk helicopter crashed at about 6.00pm (1800 GMT) today near Salthouse on the Norfolk coast," said a statement issued by the 48th Fighter Wing, which is based at nearby Royal Air Force (RAF) station Lakenheath.
A US defence official later confirmed the accident killed the four US Air Force crew members on board.
The 48th Fighter Wing response teams were on site and beginning their investigations.
Norfolk Police also issued a statement saying that next of kin will be informed before further details on the victims are released.
The force revealed that the aircraft, a highly modified version of the Army Black Hawk helicopter, may have been carrying live ammunition.
"It is not believed that anyone in the surrounding area has been injured," said the statement.
"However, there remains a 400-metre cordon around the site, which is standard for this type of incident.
"An assessment is still being carried out around the munitions which may be on the aircraft and advice from the military is being taken."
Around a dozen emergency vehicles from the fire brigade, coastguard and police remained at the scene at 11.00pm (2300 GMT).
Superintendent Roger Wiltshire, of Norfolk Police, added: "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."
The US base at Lakenheath confirmed that the chopper came during a training mission outside Cley-Next-The-Sea.
Lakenheath is situated in neighbouring Suffolk and, although it is an RAF base, hosts US Air Force units and personnel, including the 48th Fighter Wing.
Also known as the Liberty Wing, the 48th is key to US air power in Europe, and is understood to be involved in anti-terrorism operations.
Local resident Richard Kelham said it seemed the helicopter had come down in the middle of the bird reserve.
Norfolk Wildlife Trust later released a statement revealing that the accident had taken place "on the shingle bank at NWT Cley Marshes nature reserve."
"Our immediate thoughts are for the families of those who sadly lost their lives," it added.
Sue McKnestiey, who runs a shop in Salthouse, described hearing an unusual sounding helicopter pass overhead.
"I don't know about engines but I am used to the sound of helicopters and this sounded very heavy and very unusual," she explained.
"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."
Artist Rachel Lockwood added: "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."
Britain is suffering from a spell of stormy weather and high winds, but the west of the country has borne the brunt of the rough conditions.
Michael Girling, who saw the helicopter shortly before it crashed, described the conditions as "pretty mild, clear, not bad at all."
Pave Hawks are used for combat search and rescue, and are often called upon to retrieve downed aircrew in hostile environments.
During Operation Desert Storm they provided combat search and rescue coverage for coalition forces in Iraq and more than 20 were deployed after Hurricane Katrina in 2005 in support of recovery operations in New Orleans.
Today, Pave Hawks are being used in support of operations in Afghanistan, Iraq and Libya.