Troops have been involved in a massive security operation close to the border after claims that a major bomb attack had been thwarted following the discovery of a mortar device in Co Armagh.
There had been unconfirmed reports that at least two mortars had been found and the Army was continuing to hunt for another, bigger device.
But on Wednesday afternoon, police confirmed that at least one mortar had been uncovered and Army bomb experts remained at the scene to make the device safe.
Police added that the area was due to remain closed off through the night.
It is understood the mortars, which were planted vertically, could have been used to attack a Police Service of Northern Ireland (PSNI) helicopter responding to another incident.
The alert, close to the village of Cullyhanna in South Armagh, is still continuing.
Soldiers have been using sniffer dogs and metal detectors to scour hedgerows and culverts.
The finger of suspicion has been pointed at republican dissidents who have been responsible for the murders of two soldiers, a police officer and a prison guard in recent years.
Ulster Unionist MLA Danny Kennedy described the discovery as distressing.
He said: "I am concerned at both the capability and the intent of those sinister elements behind this attack and it simply reinforces how important it is that local people give total support to the police and provide them with every assistance possible as they attempt to protect and serve the entire community.
"I would appeal for anyone with any information to come forward immediately and help the police to catch whoever was responsible for this incident."
In March, a mortar attack on a police station in Londonderry was foiled after the security services stopped a van with four viable devices primed and ready to launch.
Two men were arrested and were remanded in custody after appearing before a judge in the city.
Mortars have also been found during security alerts in Belfast.
SDLP MLA Dominic Bradley said those responsible for the mortar were reckless and irresponsible.
"The people of Cullyhanna, South Armagh, and indeed the people of this island, want to be left in peace. Instead, those responsible still live in some warped existence of the past," he said.
"There is palpable anger in the local community. This community don't want this level of disruption as they go about their everyday lives.
"The community are relieved there have been no deaths or serious injury and they want those responsible to stop immediately."