Three men and one woman remain unaccounted for after a fire and two subsequent explosions at a wood flour mill.
Search-and-rescue teams are waiting to enter the collapsed four-storey building in the village of Bosley, near Macclesfield, Cheshire, as fire crews deal with a number of ongoing fires.
Police declared a major incident after the blasts.
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Fire chiefs plan to start the recovery operation later on Friday night at Wood Treatment Ltd, which was described as “a scene of devastation”.
An initial report of a fire at the premises at about 9.10am on Friday was followed by two loud explosions - which some witnesses likened to “an earthquake” - and then flames were seen jumping up to 70 metres high.
Smoke continues to drift from the scene.
Liaising with families
Cheshire police said they were liaising with the families of the four missing but were not releasing names at this stage.
Paul Hancock, chief fire officer for Cheshire Fire and Rescue Service, said: “We are still looking at four people who are unaccounted for at this stage.
“We have some urban search-and-rescue teams doing an assessment of the scene to determine whether it is safe or not to go into the property and carry out the search, and locate these four unaccounted people.
“We are still in the acute phase of the incident in that the fire crews are still dealing with fires within the building.”
He added: “The circumstances down at the scene is one of devastation. When you have two explosions in a building, the building splits open from the inside and as a result of our firefighting tactics over a course of two or three hours, it has then collapsed on itself.
“So it is extremely difficult and challenging for those fire crews and those specialist teams to go in to try and locate these potentially missing four people.
‘Big task’
“These urban search-and-rescue teams are teams that went out to Nepal and Japan recently . They have got a big task in front of them. But before we commit these crews we must ensure that they are safe to go in that environment.
“The explosion combined with a number of risks - running fuel fires, LPG cylinders, asbestos ... they are significant hazards.”
Assistant chief constable of Cheshire Police Guy Hindle said: “Irrespective of the hazards and the danger, we won’t give up and we don’t give up.”
Earlier, three other people were rushed to hospital with burns after the explosions.
One, a 29-year-old woman, suffered serious burns and blast injuries to her head, face, arms and chest, West Midlands Ambulance Service said.
She was airlifted to Queen Elizabeth Hospital Birmingham for specialist treatment.
One other person was flown by air ambulance to Wythenshawe Hospital in Manchester, and the third taken to Royal Stoke University Hospital, the North West Ambulance Service said.
A further 35 people were assessed at the scene by ambulance staff but were not injured.
The explosions at the wood treatment works shook nearby homes and emergency services warned locals to close windows and doors because of the smoke.
‘Huge bang’
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.
More than 40 pupils at Bosley Primary School, which is about a mile away from the explosion site, were kept inside the school building with the doors and windows closed.
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.
Press Association