The incident occurred on Newcastle Road, Ballynahinch, Co. Down.
One person was taken from the scene to hospital.
"Following assessment and initial treatment at the scene, one patient was transferred to the Royal Victoria Hospital, Belfast," said a Northern Ireland Ambulance Service spokesperson.
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The P.S.N.I. has closed the road.
Diversions are in place via Drumsnade Road and Old Park Road
Motorists are being advised to avoid the area.
There are no further details.