Scores of people are injured and up to 15 people have been killed after a massive explosion at a fertiliser plant in the US state of Texas.
The US Geological Survey recorded a 2.1-magnitude seismic event and the force of the blast on Wednesday was so powerful that dozens of nearby homes and buildings were destroyed.
Police say between five and 15 people have been killed and more than 160 are injured.
Emergency services officials believe ammonia may have caused the explosion at the West Fertilizer plant about 7.50pm (local time). It has been reported the company had 20 tonnes of anhydrous ammonia on site.
Firefighters had been tackling a blaze at the plant when the explosion in West, a town of about 2700 people some 32km north of Waco and 100km south of Dallas occurred, the BBC reports.
Eyewitnesses described a huge ball of fire. Officials said a five-block area in West has been levelled and there are fears people are trapped or dead inside collapsed buildings, several of which continued to burn hours after the blast.
A number of firefighters are thought to be missing. Police said the half the town had been evacuated, amid fears of possible further explosions or a leak of toxic gas.
The plant is right on the edge of town, only a few hundred metres from houses, a school and nursing home.
Sergeant William Swanton, of the Waco police department, said power to West had been shut off to prevent further accidents.
Dean Wilson, of the Texas Department of Public Safety, told a news conference that they were conducting house to house searches.
Emergency crews rushed to the scene and a triage centre was set up on a nearby football field. Police reportedly used their patrol cars to transport the injured to hospital. About 38 people admitted to Hillcrest Baptist Medical Center are said to be in a serious condition on Wednesday.
State governor Rick Perry said his office had mobilised state resources to help local authorities deal with the disaster.
Anniversary of Waco siege
The explosion comes as the United States - already on edge - pieces through deadly explosions that targeted the Boston Marathon on Monday, killing three and injuring at least 175 people and the sending of letters apparently poisoned with ricin to President Barack Obama and a senator, AFP reports.
April 19 will be the 20th anniversary of the Waco siege, a deadly confrontation between US authorities and heavily armed locals belonging to a Branch Davidian sect, led by David Koresh.
After a 51-day stand-off, a gun battle broke out with US agents and the compound was engulfed in flames. Seventy-six people were killed in an incident that many far-right groups see as a symbol of egregious US government over-reach.