Four people are believed to be trapped after an explosion and fire at a flour mills in Bosley, Cheshire, say police.
A major incident has been declared and all emergency services have gone to the site at Wood Flour Mills on Tunstall Road.
Cheshire Fire and Rescue say a four-storey building has collapsed and there is the risk of further explosions.
Fifteen fire engines are at the scene and an air ambulance has been sent.
People living nearby and a local school have been told to close their windows and doors because of smoke coming from the site, south of Macclesfield.
Student Charli Alston, 18, was walking her dog with a friend when she saw heard a "huge bang".
"It sounded like a thunder cloud," she said. "We didn't know what it was because it was sunny then. We looked over and there were massive plumes of flames. Almost as high as the hill next to it.
"I couldn't tell you how high the flames were, but I'd say a good 70 metres into the air. Huge. The sound was really really loud and there was loads of billowing black smoke.
"We phoned 999 straight away, because it was pretty terrifying watching it."
Fire officials say they were called to reports of an explosion at the mill at about 9.10am.
It said fire engines from Congleton, Bollington, Winsford, Crewe and Macclesfield attended, including an aerial appliance from Macclesfield and one from Greater Manchester Fire and Rescue Service.
North West Ambulance Service NHS Trust said it had sent "at least six ambulances, two rapid response vehicles, two doctors, four managers and the air ambulance" to the scene.
We spoke to Charlotte Alston, who lives nearby and heard the explosion take place:
35 people have been directly affected by the incident. Four have been treated at the scene for breathing difficulties and a further four have been taken to hospital for treatment – one of those is in a serious condition.
As of 3pm, four people remain unaccounted for. A safety cordon has been set up around the scene and nearby Bollin Mill Road has been shut.
Fire crews say they expect remain at the scene for some time.
Cheshire Fire and Rescue Service’s Chief Fire Officer, Paul Hancock, said: “This is a difficult and demanding incident. There are a number of hazards that make it particularly dangerous for those attempting to tackle the blaze. However, crews and our colleagues from the other emergency services continue to work tirelessly to both bring the fire under control and find the four that are missing. We urge everyone in the area to keep their windows and doors shut as there continues to be a lot of smoke in the area.”