Several thousand sheep are to be culled in an operation, co-ordinated by veterinary officials on both sides of the Border, which begins today.
The North's Minister of Agriculture, Mrs Brid Rodgers, told a press conference in Belfast that her own Department and the Department of Agriculture in the Republic would be slaughtering all sheep within, and between, the two 3km exclusion zones around farms where there have been foot-and-mouth outbreaks in Meigh, Co Armagh, and Proleek, Co Louth.
"The purpose of the cull is to eradicate any trace of the foot-and-mouth virus, which may be present in the sheep population in the areas where the two outbreaks occurred," she said.
Mrs Rodgers said her experts told her the long delay between the outbreak at Meigh and the detection in Proleek meant it was "highly unlikely" the virus had spread directly.
"It could be indirect or it could be another source. The reality is we can't say for certain," she said.
The area affected by the operation is the two 3km exclusion zones [indicated on the map] and all the land between them. The zones are demarcated by the closest roads outside the theoretical zones.
She said the Northern carcasses would be buried at a site yet to be determined within the Northern zone.
The Minister for Agriculture, Mr Walsh, said in Dundalk yesterday officials were still "desperately trying to establish the source of infection in the Cooley peninsula". Speaking at a press conference, he said it was vitally important the source was found.
He was, however, unable to confirm details about the 60 "missing" sheep referred to yesterday by Mrs Rogers.
Mrs Rodgers said her Department now believed the risk posed by a number of "missing" sheep, which had been imported into Northern Ireland and not traced, had been removed.
"The consignment that we were concerned about, we have now tracked. We had, 10 days ago, 60 that we couldn't account for; that then was reduced to 30; and we now, on foot of information from authorities in the South, feel that we have established where those 30 have gone," she said.
The information she had received from her staff and the Southern authorities was that some of the sheep had gone to an abattoir in Navan and were slaughtered immediately. Some had gone to a farm in Meath where they were also slaughtered.
Mrs Rodgers said this meant that "as far as we are concerned, we have accounted for all the sheep that came into Northern Ireland from February 1st."
Asked in Dundalk yesterday if these "missing" sheep had been among the 1,200 slaughtered at various locations in the Republic over the weekend, Mr Walsh replied: "We don't know. We have received information over the last 36 hours or so leading us to locations; whether those 60 are in that 1,200 I don't know."