Long queues have built up on the A449 following a road traffic collision on the southbound carriage close to the Usk turn-off.
Traffic is already stretching back as far as Raglan, with the road likely to be closed for much of the night.
Eye-witnesses report that emergency services are on the scene and say that the collision is thought to involve a lorry and a farm vehicle, although this is yet to be confirmed.
"It looks like a serious incident," said one eyewitness.
A second, less serious collision is also understood to have occurred between two vehicles in the queue of traffic.
Motorists are advised to seek an alternative route