Water in gas main affects 500 homes in Nantyglo
Around 500 properties are without gas in the Nantyglo area of Blaenau Gwent after water from a burst main leaked into gas pipes.
Wales and West Utilities said engineers were working to resolve the issue which it described as "complex".
The firm said its engineers were going door to door to make sure gas supplies at properties affected were turned off.
It said engineers would return to put gas supplies back on once it was safe to do so.
The firm said vulnerable customers would be given alternative heating and cooking facilities if required.