A Texas fertilizer factory exploded in a huge fireball Wednesday, flattening nearby homes and perhaps killing as many as several dozen people, with one official likening the blast to a "nuclear bomb."

Fears were mounting in the small town of West, outside Waco, that a second fertilizer tank could erupt in flames, stoking anxiety in a nation already on edge after the nerve-jangling Boston marathon bombings, which left three dead.

An apartment complex and a nursing home were destroyed, local residents flooded into emergency shelters, and at least 100 patients were hospitalized following the blast, which US seismologists said had a magnitude of 2.1.

"It's like a nuclear bomb went off," West Mayor Tommy Muska, who is also a volunteer firefighter, told CNN.

There were conflicting reports about the death toll in the chaotic aftermath of the explosion in the southern US state, which witnesses said sent a huge fireball into the air.

A toll of up to 60-70 dead was initially reported by KWTX television, citing the director of West emergency services, George Smith.

Keith Hopkins, an administrator with Providence hospital in Waco, told AFP that emergency personnel in West had also given him the figure of 60 to 70 fatalities.

A spokesman for the Texas Department of Public Safety, D.L. Wilson, told reporters he could not "confirm or deny" the figure, saying: "We have confirmed fatalities. The number is not current yet. It could go up by the minute."

House-by-house searches were being conducted to find any additional victims, Wilson said.

The United States was already on edge in the wake of Monday's deadly twin bombings in Boston, and a scare in Washington over mail apparently laced with the poison ricin sent to President Barack Obama and a US senator.

The huge blast also came just before the 20th anniversary on Friday of a deadly confrontation in Waco between federal authorities and heavily armed members of a religious group, the Branch Davidians.

The explosion at the West Fertilizer plant, sparked by an enormous blaze, occurred just before 8:00 pm (0100 GMT), Waco Assistant Fire Chief Don Yeager told AFP by phone.

The cause was not immediately known but Yeager said it was an anhydrous ammonia explosion. Officials expressed fears that toxic fumes could settle over the town.

Power and gas has been cut to some areas of the town as a precaution, Waco police sergeant W. Patrick Swanton told CNN.

Mark Felton, executive director of the Waco-based Heart of Texas Red Cross, told AFP that people were "flowing into the shelters" set up for evacuees and those whose homes were destroyed, without providing a specific figure.

"There are hundreds of emergency response vehicles lined up," Felton said.

Witnesses said they were stunned by the sheer force of the blast.

"It knocked me down, it knocked me back. It was like the whole road just picked up," resident Cheryl Marich, whose home was destroyed and whose husband was fighting the blaze, told CNN.

Jessica Turner, a geophysicist at the US Geological Survey, told AFP experts "were able to see the ground motion that the explosion created," putting the magnitude of the shockwave at 2.1.

There were growing fears that a second fertilizer tank at the plant could explode, widening the disaster that Muska said had leveled up to 80 homes in the small Texas town of 2,500 people.

The mayor told CNN that six or seven firefighters who had attempted to tackle the blaze were unaccounted for.

"It exploded just like the Oklahoma City bomb," Jason Shelton, a clerk at the Best Western Czech Inn in West, told The Dallas Morning News, referring to the 1995 bombing that killed 168 people.

"I live about a thousand feet from it and it blew my screen door off and my back windows. There's houses leveled that were right next to it. We've got people injured and possibly dead."

Another witness, Bill Bohannan, told the Waco Tribune-Herald: "It knocked us into the car... Every house within about four blocks is blown apart."

In the 1993 Waco siege, following a 51-day standoff, the group's compound burned down after an assault was launched.

Dozens of people were killed in an incident that many far-right groups see as a symbol of egregious US government overreach.