Large early-morning "explosions" rocked the the Port Talbot steelworks in South Wales.
South Wales Police confirmed they are in attendance at the TATA-owned plant, which is the largest steel works in the UK.
Locals described red smoke and the smell of sulphur after the loud bangs.
Helicopters were heard overhead as the emergency services rush to the scene, with one resident saying "I can see the fire."
One local tweeted soon after the blast at 3.30am: "Huge explosion in Port Talbot just woke me up, the house shook for a good while and I actually thought it was an earthquake again."
Another said: "I swear something just blew up in port Talbot. 3 huge bangs and the pressure blew my bedroom door open."