"Four crew members were on board at the time of the accident," Lieutenant Steve Curry of the US naval air station at Sigonella told AFP.

"Firefighters and security from Sigonella NAS are at the scene of the accident. The status of crew members is unknown."

"The names of the crew members are being withheld until next of kin are notified." He said the cause of the accident was under investigation.

The MH53-E Sea Dragon helicopter crashed on land about 10 mileskilometres) southwest of Sigonella at around 5:00 pm (1500 GMT) near Palagonia in the Ramacca region of Sicily, Curry said.

A spokesman for the Carabinieri regional command told AFP earlier that attempts to rescue the crew members had been thwarted by the flames which engulfed the aircraft.

"We think there are three or four people in the aircraft, but the flames are too high and are preventing us from getting close at the moment," the Carabinieri officer said by telephone.

A witness, a local farmer, told Italy's ANSA news agency that the aircraft appeared to veer off course and trail black smoke before crashing.

Curry said the helicopter was attached to the HC4 unit at Sigonella, which provides heavy helicopter combat support to the US fleet in the Mediterranean.

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