Four crew members have died after a US military helicopter crashed in Norfolk, England on a training mission.

The Pave Hawk was taking part in a low-flying exercise when it came down in Cley-next-the-Sea, in the north of the county.

Residents told of hearing a "heavy and very unusual" sound overhead as the helicopter - which specialises in recovering troops from war zones - plummeted into marshland.

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

A 400m cordon protected the crash site, which was at a nature reserve, and residents were allowed to stay in their homes, although pedestrians and motorists were cleared from the scene.

Emergency services, including the police, fire and ambulance - as well as the RNLI, which were later stood down - were called at around 7pm after reports of a crash in the rural village.

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 80km from the crash site.

The 48th Air Wing of the US Air Force 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."

A spokesman for the base later said: "The aircraft, assigned to the 48th Fighter Wing, was on a low-level training mission when the crash occurred."

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.

Cley is one mile east of Blakeney and four miles north of Holt, on the main coast road between Wells and Sheringham.

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."

Norfolk Wildlife Trust said on its website it was "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.

Superintendent Roger Wiltshire, of Norfolk Police, 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."

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

Mr Wiltshire said investigators would "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, he confirmed.