A news release from police in Norfolk said a U.S. Air Force Pave Hawk HH-60G helicopter from a British base in Lakenheath had crashed and they believed the four people aboard had died.
The Pentagon had no update on the official status of those on board.
Police said a large area had been cordoned off as authorities responded to the scene.
The Facebook page for the RAF base said it was a training mission.
CNN was first alerted to reports of the crash by Twitter messages from the region.
Cley is 180 kilometers (about 110 miles) northeast of London.
According to the Air Force website, a Pave Hawk is a “highly modified” version of a Black Hawk helicopter that often carries a crew of two pilots, one flight engineer and one gunner.
CNN’s Barbara Starr contributed to this report.
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