Four people are feared trapped and three others have been rushed to hospital with burns after several explosions at a mill.
Police declared a major incident after the blasts at Wood Flour Mills in the village of Bosley, near Macclesfield, Cheshire.
Fire crews said the incident involved the collapse of a four-storey building and warned there was a risk of further explosions.
"There is a fully developed fire in several areas," a Cheshire Fire and Rescue spokesman said.
"Firefighters are facing a dangerous situation as they cannot rule out further explosions. People are advised to avoid the area.
"It is believed at this stage that four people are trapped."
A 29-year-old woman suffered serious burns and blast injuries to her head, face, arms and chest, West Midlands Ambulance Service said.
She has been airlifted to Queen Elizabeth Hospital Birmingham for further specialist treatment, while two other people who suffered burns have also been taken to hospital.
One person was flown by air ambulance to Wythenshawe Hospital in Manchester, with the other taken to Royal Stoke University Hospital, the North West Ambulance Service said.
A further 35 people have also been assessed at the scene by ambulance staff but were not injured.
The explosions at the wood treatment works shook nearby homes and emergency services have warned local residents to close windows and doors because of smoke in the area.
Emergency crews were initially called to reports of an explosion at the site at around 9.10am. Cheshire Police confirmed later there had been a further two blasts.
Student Charli Alston, 18, who heard the first explosion as she walked her dog,said: "I was walking down my drive and there was just a huge bang. It sounded like a thunder cloud, 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."
Charlotte Maher, who lives close to the scene of the explosions, said the incident was a "major devastation" for the village.
She told Sky News: "The house completely shook. We thought it was an earthquake.
"Black smoke started coming out and we heard the sirens.
"Because Bosley is such a small village, a lot of people we know work there.
"It's a major devastation for the village.
"We felt two vibrations here five minutes down the road. I can't imagine how the houses down there took it."
Fifteen fire engines have been dealing with the aftermath of the explosions, Cheshire Fire and Rescue said
A spokesman for North West Ambulance Service NHS Trust said: "Our primary role is to assess the level and nature of injuries, and ensure that those people most in need are treated first and taken to hospital.
"At this stage, it is too early for us to confirm the number of casualties."
More than 40 pupils at Bosley Primary School, which is about a mile away from the explosion site, were being kept inside the school building with the doors and windows closed.
The school is due to close for the summer holidays today, a spokesman said.
The incident in Cheshire comes after two men were killed in an explosion at an industrial unit in Norfolk on Monday.
Daniel Timbers, 29, and Barry Joy, 56, both died in the blast at digger bucket manufacturer Harford Attachments in Norwich, which is believed to have been caused by a fireball resulting from a build-up of toxic fumes.