Householders in Co Tyrone were warned to take precautions today after abnormally low chlorine levels were detected in drinking water.
Northern Ireland Water has advised customers in the Gortin, Newtownstewart, Plumbridge and Strabane areas to boil all tap water before using it.
The warning comes after checks on the quality of water being piped from the Killymore reservoir raised concern.
A NI Water spokeswoman said the advice was only precautionary.
She said a flushing of the system has begun and the chlorine levels throughout the system are being checked.
Further testing will continue until samples have returned to normal.
Customers will be notified as soon as their water can be used without boiling, she added.
PA