A teenage boy is in hospital after being knocked off his bike and dragged 40 yards under a four-wheel drive vehicle in Sydney's southwest.

The 14-year-old suffered head injuries, a fractured skull and severe abrasions when he was knocked down at Canley Vale, just before 8:30 p.m. on Tuesday, the ambulance service said.

He was then dragged for about 40 yards under a four-wheel drive vehicle before it was able to stop.

An ambulance crew and police went to the scene of the accident, while an NRMA CareFlight trauma team managed to land in a nearby park.

A doctor administered the boy with a general anaesthetic, then placed him on a ventilator to control his breathing.

He was taken by road to The Children's Hospital at Westmead, where he was in a critical condition when he arrived.

Police investigations of the incident are continuing.

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