Troops were involved in a massive security operation close to the border in Northern Ireland after claims that a major bomb attack had been thwarted.
There were unconfirmed reports that at least two mortars had been found and the Army was continuing to hunt for another, bigger device.
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, has been continuing for several days.
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.
As yet the PSNI has declined to make a statement.
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.