The remnants of the fertilizer plant burn after an explosion on Wednesday in West, Texas. The blast, triggered by a fire of unknown origin at the plant, registered as a magnitude-2.1 seismic tremor that was felt in Dallas, 130 km to the north, and beyond. Mike Stone / Reuters

Firefighter deaths

Among those believed to be dead are group of volunteer firefighters and a single law enforcement officer who responded to a fire call at the plant about an hour before the blast. They remained unaccounted for on Thursday morning.

The toll included 50 to 75 houses, an apartment complex with about 50 units that one state police officer said was reduced to "a skeleton", a middle school and the West Rest Haven Nursing Home, from which first responders evacuated 133 patients, some in wheelchairs.

Other witnesses compared the scene to the 1995 Oklahoma City bombing, and authorities said the plant made materials similar to those used in the bomb that tore apart that city's Murrah Federal Building.

Authorities said it will be some time before they know the full extent of the loss of life, and West Mayor Tommy Muska told reporters that his city of about 2,800 residents needs "your prayers".

"We've got a lot of people who are hurt, and there's a lot of people, I'm sure, who aren't gonna be here tomorrow," Muska said. "We're going to search for everybody. We're going to make sure everybody's accounted for. That's the most important thing right now."