An explosion at a waste water treatment works near Bristol that has caused multiple casualties is believed to have involved one of its chemical tanks, police have said.
A major incident was declared at Wessex Water's Bristol water recycling centre in Kings Weston Lane, Avonmouth, after reports of a "large explosion" at 11.22am on Thursday.
Avon and Somerset police said inquiries are ongoing into the exact cause of the blast.
A spokesman for the force said: "An explosion is believed to have occurred involving one of the chemical tanks at Wessex Water's Bristol water recycling centre.
"A rescue operation was subsequently led by the fire service, involving five appliances and two turntable crews, and we can confirm there have been a number of casualties.
"A full investigation into the incident will be conducted."
Chief Inspector Mark Runacres said: "Officers remain at the scene and are likely to be for some time as we work together with other agencies in dealing with this major incident.
"A cordon has been set up and we'd advise members of the public to avoid the area at this time where possible to allow the emergency services to carry out their work.
"Police inquiries into exactly what happened are at a very early stage and are continuing."
Avon Fire and Rescue Service said crews from eight stations were mobilised to respond to the incident and confirmed there had been "multiple casualties on site".
A spokesman for South Western Ambulance Service said its land, air, and other specialist paramedic teams were also deployed.
Witness Jawad Burhan took a photo showing a tank that had exploded after the blast.
He told the PA news agency there was a "helicopter looking for missing people" and police had closed a nearby road leading up to the building, which he believed was a waste centre.
"I heard the sound, I'm working beside the building in another warehouse," he said.
"After 10 minutes I saw the helicopter coming and the police."
Darren Jones, MP for Bristol North West, said: "This is a serious incident and my thoughts are with those workers who have injured on site today.
"I'm grateful to our fire, police and ambulance staff for their quick response."
Bristol mayor Marvin Rees said on Twitter: "There has been a large explosion at the Wessex Water site on Kings Weston Lane in Avomouth.
"Thank you to @AvonFireRescue @ASPolice and @swasFT who are doing their very best to help all those involved.
"My thoughts are with anyone affected by this incident."
Home secretary Priti Patel said in a tweet: "Deeply concerned at reports of a warehouse explosion in Avonmouth.
"My thoughts are with those affected and our brave emergency services working hard to resolve this incident."