Four people were feared dead after a helicopter crashed in eastern England on Tuesday, police said, with local media reporting that it was believed to be a US military aircraft.

The crash occurred in the Cley area of the north Norfolk coast, police said in a statement.

"There are believed to be four fatalities," the statement added, saying a 400 metre (0.25 mile) area had been cordoned off.

Sky News, citing sources, said the helicopter was believed to be a US Black Hawk flying from the nearby US airbase RAF Lakenheath.

"We are working on an incident," a spokesman at the airbase said, adding he had no further details.