Fishery officers are investigating a pollution incident, which killed fish in the Mullaghduff River, north of Castleblayney in Co Monaghan.
A spokesman for the Eastern Regional Fisheries Board said it appeared that the pollution had occurred over the past few weeks but because it was in a remote area, it had only been detected by the board on Wednesday evening.
There were reports that hundreds of fish had been killed but the spokesman said that had not been confirmed by fisheries officers at the site.
Initial investigations indicated that the pollution had occurred north of the Border.
The Mullaghduff River is a tributary of the Fane River and crosses the Border to enter Lough Muckno.
The Eastern Regional Fisheries Board is working with Monaghan County Council and the Fisheries Conservancy Board for Northern Ireland to locate the source of the pollution.
Last night, oxygen measurements were being taken to see if organic pollution, such as effluent, had caused the incident.
The Mullaghduff River has had pollution problems in the past and prosecutions have been brought in a number of cases.