A lorry overturned near the Hardwick roundabout this morning, causing major tailbacks on the Heads of the Valleys route.
A fire service spokeswoman said the incident, involving one vehicle, has been dealt with.
Drivers at the scene have reported slow moving traffic.
Daniel Seymour, 34, was working in the area when the lorry overturned. The highway maintenance employee for Monmouthshire County Council said:
“I was coming up toward the Hardwick roundabout. I was seconds away from going right underneath the lorry.
“Me and my colleague Mark got out of our van climbed into the lorry cab, and turned the engine off to make everything safe. There was diesel spilling everywhere.
“We checked on the driver, he was pretty dazed but okay. We gave him five minutes and then dragged him out of the cab. He’s okay now. Just a cut on his ear from an earpiece he was wearing. He’s gone to Nevill Hall Hospital.
“If I was two seconds earlier, we would have been underneath the lorry as it turned over.”
A police spokeswoman said the incident was being dealt with, and motorists should expect delays.
"Emergency services were called at around 9.26am this morning to reports of an overturned lorry on the Hardwick Roundabout near Abergavenny. The driver of the lorry received minor injuries and has been taken to hospital as a precaution.
"Officers have advised that the road is blocked but passable and drivers in the area should expect delays. Diversions have been put in place and drivers are advised to follow diversion signs where possible."
If anyone has any information about this, please call 101 quoting log 103 10/5/17