Gardaí in Co Monaghan are investigating reports of an alleged incursion by a British army helicopter into an area near Castleblaney.
The helicopter is understood to have travelled low over houses in the Donaghmoyne area at around 7.15 p.m. yesterday evening.
A Garda spokesman could not confirm whether the force had been in contact with the Northern authorities over the alleged incursion, saying simply the matter was being investigated.
But a British army spokesman said they had been contacted about the incident by a member of the gardaí stationed at Hackballs Cross in Dundalk, Co Louth.
However he denied a report that the PSNI had apologised on its behalf for the incident.
He said the army has no record of such an incident and if it did happen it was most likely to have been an accident.
He said such incidents can happen as a result of an error on behalf of the pilot.
South Monaghan Sinn Féin county councillor Mr Matt Carthy described the alleged incursion as an "outrage", calling on the Government and the Minister for Foreign Affairs, Mr Cowen, "to make an immediate and forceful protest to the British government".
Mr Carthy said: "Last night at least one British army helicopter crossed the Armagh- Monaghan Border at Corrinshigagh, Cullaville.
"To do so they had to cross the River Fane which is a very visible indication of county boundaries.
"I have received a number of phone calls this morning from irate residents informing me that the military helicopter travelled up to two miles into the heart of the parish of Donaghmoyne in County Monaghan.
"The illegal incursion lasted for at least fifteen minutes.
"The British army have no right to be in any part of Ireland. At this stage in the peace process to have these heavily armed military helicopters crossing the border is an outrage."