Several people are feared dead and many more injured after an explosion at a fertilizer plant in West, Texas, Wednesday night.
Police from the small town of West, about 20 miles north of Waco, said between five and 15 people were killed, though those figures remain an estimate and could yet rise.
Emergency services are going door to door to search for possible casualties. More than 160 people are known to have been injured.
"I've never seen anything like this," McLennan County Sheriff Parnell McNamara told Reuters. "It looks like a war zone with all the debris."
Reports are that firefighters responded to a small blaze at the plant Wednesday evening; a short time later, there was an explosion so violent it rocked neighboring communities up to 50 miles away.
At least three first responders were still missing by Thursday morning.
The explosion at the West Fertilizer plant occurred just before 8 pm local time, Waco assistant fire chief Don Yeager told AFP by phone.
There are "no indications this was anything other than an accidental fire," Sgt W. Patrick Swanton of the Waco police department told a press conference early Thursday morning.
Investigators will nonetheless treat the blast site as a crime scene until they know for certain, he said.
Half the town was evacuated as a precaution against further explosions or toxic gas leaks, though Swanton said that as of Thursday morning there was no immediate danger.
President Barack Obama has been notified and is "closely monitoring" the situation via the Federal Emergency Management Agency, which is in touch with local authorities, a White House official told ABC News.
Fire departments from nearby jurisdictions rushed to the scene just outside West, where the blast was so powerful that it knocked down some nearby buildings, and set fire to others.
At least 50 buildings were badly damaged, including an apartment complex that was left like "a skeleton standing up," said D.L. Wilson of the Texas public safety department.
A "significant area" around the fertilizer plant has been destroyed, W.P. Swanton, the Waco police spokesman, said.
"I don't know how many folks may still be trapped in rubble."
According to the US Geological Survey, the explosion registered as a 2.1-magnitude seismic event. Videos showed a huge fireball shooting into the sky, accompanied by a massive boom.
At the plant, the fire was reportedly smoldering hours after the blast.
"It was a small fire and then water got sprayed the ammonia nitrate, and it exploded just like the Oklahoma City bomb," witness Jason Shelton told The Dallas Morning News.
There are so many injured, crews are using a nearby football field as triage, the Waco Tribune-Herald reported.
Injuries include "blast injuries, orthopedic injuries (and) a lot of lacerations" from flying debris, doctors told CNN.
"I said, ‘This thing is going to blow’... and I told my mom and dad to get in the car," witness Bill Bohannan told the newspaper. "I was standing next to my car with my fiancee, waiting for my parents to come out and (the plant) exploded. It knocked us into the car... Every house within about four blocks is blown apart."
There are also fears that shifting winds could spread an anhydrous ammonia gas cloud, the Los Angeles Times reported.
Agence France-Presse contributed to this report.