As many as 15 people are believed to have been killed and over 160 injured after an explosion ripped through a fertiliser plant in Texas.
More than half of the town of West, near Waco, has been evacuated and people are still being pulled from damaged buildings following the blast.
The force of the explosion had a 2.1 magnitude, according to the US Geological Survey, equivalent to a minor earthquake.
Paul Earle from the organisation told Sky News the blast would have felt much bigger than 2.1 because the explosion was on the surface unlike an earthquake which happens underground.
And a lot of energy would have been released/lost into the air.
The blast was so powerful that a nearby block of flats was destroyed and 130 residents of a local nursing home were injured.
Firefighters were tackling a blaze at the plant when the explosion happened in West, which has a population of about 2,800 people. A small group of firefighters are among those missing.
Sergeant William Swanton, from Waco Police Department, said the death toll was "anywhere from five to 15 at this point" but could rise.
He said there were scenes of "extreme devastation", adding: "We're going house to house, business to business, and we're seeing quite a bit of devastation in the area of the plant".
"They're still pulling victims out, still bringing victims to triage. There may be firefighters that are unaccounted for and potentially a law enforcement officer as well."
He went on to say locals "are going to be in a state of recovery for a very long time".
He also said that a helicopter which was helping with the rescue operation was damaged.
The blast damaged as many as 75 homes, as well as a local school.
A number of people are also suffering from "respiratory distress due to chemical inhalation".
The explosion at the West Fertiliser Company happened shortly before 8pm local time on Wednesday and could be heard as far away as 45 miles.
A man filmed the initial fire, and captured the moment of the explosion on camera.
His child is heard shouting: "Dad, I can't hear, let's get out of here. I can't hear anything." The dad says simply: "Oh my god."
Waco Assistant Fire Chief Don Yeager said it was an anhydrous (without water) ammonia explosion.
Anhydrous ammonia is a nitrogen-hydrogen gas widely used as a fertiliser, but it is also a key component of many explosive devices.
West's mayor Tommy Muska, who is part of the team of local volunteer firefighters, said a number of his colleagues are unaccounted for.
He told CNN: "It's like a nuclear bomb went off."
In a statement, President Barack Obama offered the prayers of the nation to the people of West.
"A tight-knit community has been shaken, and good, hard-working people have lost their lives," he said.
And Texas Governor Rick Perry said: "Our thoughts and prayers are with the people of West, and the first responders on the scene."
Debby Marak told The Associated Press she noticed a lot of smoke coming from the area across town near the plant, which is near a nursing home.
She said she drove over to see what was happening, and when she got out of her car two boys ran towards her screaming that officials told them to leave because the plant was going to explode.
Moments later the blast happened.
"It was like being in a tornado," the 58-year-old said. "Stuff was flying everywhere. It blew out my windshield. It was like the whole earth shook."
Police officers have reportedly been transporting the injured to local hospitals in their patrol cars.
As many as a dozen helicopters have been sent to the West High School stadium where ambulances are waiting to transport victims to hospitals.
Glenn Robinson, chief executive of Hillcrest Baptist Medical Centre, in Waco, told CNN his hospital had received 66 injured people for treatment, including 38 who were seriously hurt.
He said the injuries included blast injuries, orthopedic injuries, large wounds and a lot of lacerations and cuts.
American Red Cross crews from across Texas are being sent to the site.
The number of people arriving in the town offering assistance has become a logistical problem in itself, emergency workers say.
They are also anticipating further disruption later, with heavy thunderstorms and potential tornadoes forecast in the area.
The explosion comes on the eve of the 20th anniversary of the Waco siege - a deadly confrontation between federal authorities and heavily armed locals.
The Dallas Morning News said that the fertiliser company previously reported to the US Environmental Protection Agency and local public safety officials that there was no risk of fire or explosion at the plant.