Four people died on Thursday when a chemical tank exploded at a waste water treatment plant near Bristol in western England, police said.

A fifth person was wounded in the explosion at the plant in Avonmouth, but their injuries were said to not be life-threatening.

Police said they would not be speculating on the cause of the blast but added it was not being treated as terror related. 

Avon and Somerset Police Chief Inspector Mark Runacres said that the explosion at the plant operated by Wessex Water occurred in a silo that holds treated bio-solids before they are recycled as organic soil conditioners.

Witness Jawad Burhan said there was a "helicopter looking for missing people" and that police closed a nearby road leading up to the plant.

"I heard the sound, I'm working beside the building in another warehouse. After 10 minutes I saw the helicopter coming and the police," he said.

Emergency services were called to reports of a large explosion at approximately 11:20am (1120 GMT), with fire services leading the rescue operation.

Luke Gazzard, from Avon Fire and Rescue Service, said the incident had been "very challenging".

"Luckily our local fire station is not too far from here so we had crews in attendance very, very quickly," he said. "It is a terribly sad incident."

Police said three of the victims were employees of Wessex Water, while the fourth was a contractor.

Prime Minister Boris Johnson tweeted that he was "deeply saddened to learn that four people have lost their lives". 

"Our hearts go out to the victims and their families," he added.

Another witness, Kieran Jenkins, told the BBC he was in a nearby warehouse when the explosion occurred, saying it was "shaking and we literally stood there in shock.

"Next thing you know we looked out of the windows and all we could see was people running.

"We don't know what happened. It was a bit of a shock really. I heard a bang... we didn't know what was going on," he added.

Police reassured local residents that there was "not believed to be any ongoing public safety concerns" following the blast but urged members of the public to avoid the area. 

Local MP Darren Jones tweeted: "I'm pleased that the situation has been contained and that there is no further risk to local people".

"There will be a full investigation taking place but, for now, we pay our respects to those who have suffered and lost their lives today," he added.

A probe has been launched into the blast with the Health and Safety Executive.