Twenty-seven thousand ducks have been culled at a farm in Co Tyrone amidst ongoing concerns about the presence of avian flu in commercial flocks on both sides of the border.
Northern Ireland's Chief Veterinary Officer Robert Huey said it had been done as a "precautionary measure" following tests which indicated a suspected outbreak of the poultry disease near Aughnacloy.
The cull of the ducks follows several confirmed cases at poultry businesses just across the border in Co Monaghan.
The disease has also been found in wild birds found dead in Northern Ireland in recent weeks.
Northern Ireland has an extensive poultry industry.
A temporary control zone has also been put in place around the affected premises pending the results of confirmatory tests.
That will mean tighter controls on the movement of poultry and enhanced record keeping and biosecurity measures for other similar businesses inside that zone.
Mr Huey urged anyone who had poultry, including those who keep birds as a hobby, to get registered with the authorities.
"I am extremely concerned about the serious risk of spread and this is a very worrying development," he said.
Bird flu is an annual risk brought by migratory wild birds.
If it gets into commercial flocks it can cause rapid illness and death, and can affect trade in poultry meat and other products.