A helicopter has crashed on to a highway in the US state of Maryland and burst into flames, killing all four people on board, officials say.

No vehicles were hit when the commercial aircraft came down on Interstate 70 in western Maryland.

Firefighters found the helicopter engulfed by fire by the time they arrived at the scene on Thursday night.

Visibility was said to be limited because of fog, but it was not clear if weather played a part in the crash.

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