A slurry spillage of more than 400,000 gallons has polluted the River Ballinderry.
Three miles of river may have been affected after the slurry escaped from a farm in the Cookstown area.
The environmental impact has been assessed as high.
The source of the spill has been identified but will not be revealed as a Northern Ireland Environment Agency (NIEA) investigation is continuing.
“I am concerned that such a large amount of slurry has polluted the local environment here,” said Environment Minister Mark H Durkan.
“The important thing now is for the problem to be addressed quickly and professionally.”