Blaze at Pure Wafer Limited's Swansea industrial unit
Police are advising residents to keep their windows and doors shut due to a fire at a large industrial building on Swansea enterprise zone.
An area around Pure Wafer Limited has been sealed off and there are chemicals and cylinders at the scene on Millbrook Drive, Llansamlet.
The fire service said they are protecting a "large oxygen and nitrogen store".
Up to 70 firefighters have been at the scene since 9:01 GMT on Sunday.
The precautionary warning over windows and doors applies to residents in Birchgrove and Glais.
Roads in the immediate vicinity of the business park are also closed and members of the public are asked to avoid the area.
Mid and West Wales fire service confirmed they are pumping water from the nearby River Tawe.
A spokesman said: "The fire has gone through the roof and spread to the walls and the voids."
Natural Resources Wales said officers are at the scene working with the fire service to minimise environmental impact from the incident.
Inspector Andy Walbeoff, of South Wales Police, said: "We're not looking for anyone inside, we are just dealing with the fire itself."
Chris Davies, 26, a Royal Mail worker of nearby Tregof village, said he heard an explosion at his home, half a mile from the site.
He said: "There was a large explosion and the sound of gas escaping, a hissing noise.
"They have cordoned off the road, but I could see there were four storage tanks and it looked like one of them had exploded.
"There was nothing left of it. The firemen were on a cherry picker tacking it."