A woman has been trapped and a man injured after a serious head-on crash between a sedan and a semi-trailer on a highway in the Blue Mountains.

The vehicles collided on Darling Causeway, near Cobb Lane at Mount Victoria, about 11.30am on Thursday.

The 25-year-old woman behind the wheel of the sedan was trapped for a short time before being freed.

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She was flown to Westmead Hospital in a serious condition.

Her passenger, a 22-year-old man, was treated at the scene before being taken to hospital for further treatment.

The truck driver, a man aged in his 30s, was not injured but was taken to hospital for mandatory testing.

A man was also injured. Credit: 7NEWS

Footage captured by the 7NEWS helicopter shows the sedan left with extensive damage to its front and the driver’s side.

Six ambulances and two helicopters were sent to the scene.

Darling Causeway has been closed in both directions between the Great Western Hwy and Bells Line of Rd/Chifley Rd following the crash.

Motorists have been advised to avoid the area or delay their journey.

Investigations into the crash are underway.