Residents said the smoke had caused a “fogginess” and “acrid smell” stretching for several miles between Torfaen and Pontypool.
SWFRS warned residents to keep windows and doors closed, and advised commuters to switch air conditioning to recirculation mode in their vehicles.
The fire service said it had cut a full perimeter line around the incident.
“This offers a line of protection with the aim of keeping the incident contained, supported by two additional water bowsers from partners and contractors,” it said.
North Wales Fire and Rescue Service said crews continued to monitor the situation in Capelulo, near Sychnant Pass, in the wake of a large grass fire on Conwy Mountain.
Elsewhere, firefighters have tackled fires at Hope Mountain and Brymbo, in Wrexham county, Pentrefelin, Holyhead, and Pentre Halkyn overlooking the A55 in Flintshire.
The fire service said they were continuing to oversee conditions as a precaution, with teams from Mid and West Wales Fire Service on hand to help.