A woman has been air-lifted to the Heath hospital in Cardiff with serious leg injuries, following a crash on the A40 this afternoon.
Emergency services were called to the crash, involving two vehicles, near the Gibraltar Tunnels in Monmouth just after midday.
One of the cars left the road, falling down an embankment into trees. It was driven by a man in his fifties, with a woman and two children as passengers.
The man and children were taken to Neville Hall Hospital with minor injuries
The other car is believed to be a black Ford Fiesta or Ford Focus, being driven by a white man aged between 20-30 years old.
The car was last seen turning left at the traffic lights into Monmouth.
The northbound A40 has been closed and diversions are in place.
Police are asking anyone with information to come forward.