Three ambulances and a fire crew are at the scene.
Four police vehicles have been deployed to divert traffic at both ends of the collision on the main road between Carryduff to Ballynahinch.
The road has been closed between the Temple crossroads and the Bresagh Road.
The accident involved two small black cars who have appeared to have been involved in a head on collision, says one eyewitness.
A motorist who was diverted off the scene told Belfast Daily: “The scene is just horrific.
“It would appear that this was a head on collision. The two cars are completely crumpled at the front.
“Paramedics and fire crews were tending to the people in both vehicles.
“It will be a miracle if somebody was not seriously injured.”
Diversions are in place.
Road users are advised to seek an alternative route, if possible.