The bodies of four airmen killed when a US Air Force helicopter crashed in eastern England are unlikely to be recovered until Thursday, police said on Wednesday.

The HH-60G Pave Hawk chopper, based at the US-run Lakenheath air base, crashed at a nearby nature reserve in Cley-next-the-Sea, Norfolk, while flying low on a training exercise on Tuesday evening.

Emergency personnel joined US and British air force officials working through the night at the scene of the crash, which cast debris over an area of marshland the size of a football field, Chief Superintendent Bob Scully of Norfolk Police said.

"We are securing all the necessary evidence in order to find out what happened to the aircraft," Scully told journalists.

"There are a significant number of specialists from the Royal Air Force, from the United States Air Force and senior investigating officers and crime scene investigators from Norfolk Constabulary all working together."

The bodies of the airmen had not yet been removed from the wreckage because of the need to preserve the evidence, Scully added.

"It will take some time to remove the deceased from the aircraft," he said, adding that it was too early to say what had caused the crash.

It took place at the reserve offering spectacular views of the coast and a popular spot for bird-watchers.

The helicopter was carrying live ammunition, and although Scully said this was "not of any great significance", police restricted access to the nature reserve because of bullets scattered over a large area.

The US 48th Fighter Wing, based at Lakenheath, said the loss of the four airmen was being "felt deeply" at the base.

"I can only imagine the hurt and sorrow felt by the family and friends of these airmen," Colonel Kyle Robinson, the unit's commander, said in a statement. "You are in our hearts and minds."

Helicopter flew 'very fast and very low'

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.

The Pave Hawk is a modified version of the Black Hawk helicopter.

Sue McKnespiey, a shopkeeper who lives near the crash site, said she heard the helicopter flying "very fast and very low" overhead.

"I am used to the sound of helicopters, and this sounded very heavy and very unusual," she said.

"My gut instinct was there was something wrong."

High winds and torrential rains have battered swathes of Britain in the last few weeks, but the east of England has not borne the brunt of the rough conditions.

Michael Girling, who saw the helicopter shortly before it crashed, described the conditions at the time 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 used in military operations in Afghanistan, Iraq and Libya.

The Norfolk crash comes a month after a police helicopter crashed into a busy pub in Scotland's biggest city Glasgow, killing ten people.