A bomb alert on the Belfast to Dublin road at Newry is likely to be a long and complex operation, police have warned.
The alert began shortly before 2300 BST on Thursday after two bomb warnings were received.
A suspicious van was later found in an underpass.
The dual carriageway remains closed in both directions, as does the railway line that runs alongside.
Earlier, police warned motorists who had ignored or moved traffic cones close to the scene of the alert that they had a "blatant disregard" for people's safety.
They said the vehicle was "cordoned off", but some motorists had been able to get close.
Motorists have been advised to use the diversions in place.
BBC Northern Ireland reporter Conor MacAuley said that police had told him the suspect vehicle was "extremely well cordoned off".
However, speaking on Friday morning, he said some motorists had still been able to get close to the vehicle.
"There are road closed signs up, there are cones, some signs and lights, but I can't see any police presence at the moment," he added.
"What has happened is that someone has come along and they have moved the cones out of the road and traffic is moving freely up and down past the suspect vehicle.
"I have seen dozens of cars driving past this morning, some of them with small children in the back of their cars doing the school run.
"People obviously don't realise the potential danger they are in."
He said police would be aware of the dissident republican threat in the area.
"It is not long ago that a primed mortar was found on the motorway here which led to the discovery of a bomb-making factory in County Louth," he said.
"Police will be cautious because they will know that if this was called in with very specific details about where this suspicious vehicle was there is always the risk there is the possibility of a secondary device.
"I would say they will move very slowly and this alert could go on for some time yet."
The railway line between Newry and Dundalk has also been closed because of the ongoing security alert.
Passengers are being transported by bus between these stations.
There were also two hoax bomb alerts in Lurgan earlier.