A Texas fertilizer factory exploded in a huge fireball Wednesday, destroying many nearby homes and killing between five and 15 people, with one official likening the blast to a nuclear bomb.

Smoke and an acrid smell of burning lingered in the air hours after the blast in the small town of West, near Waco, and officials expressed fears that toxic fumes could settle over the area.

There was also concern that a second fertilizer tank could explode, stoking anxiety in a nation already on edge after the Boston marathon bombings which left three people dead.

The cause of the blast -- which destroyed an apartment complex and a nursing home and sent residents fleeing into emergency shelters -- was not immediately known.

"It's like a nuclear bomb went off," West Mayor Tommy Muska, who is also a volunteer firefighter, told CNN of the explosion, which witnesses said sent a huge fireball into the air.

The death toll "is estimated anywhere from five to 15 at this point," while three area hospitals had treated more than 160 people with varying injuries, Waco police sergeant W. Patrick Swanton said, warning that the toll could rise further.

Muska said six or seven firefighters were unaccounted for and that around 80 homes had been leveled .

The disaster comes with the United States jittery following 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.

It also came just ahead of 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.

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 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."

A spokesman for the Texas Department of Public Safety, D.L. Wilson, told reporters house-by-house searches were being conducted in the hunt for victims.

West, with a population of about 2,800 people, is home to a thriving Czech community dating back to the late 1800s, as immigrants settled the American frontier.

In Prague, the foreign ministry said its ambassador to Washington, Petr Gandalovic, was traveling to West to study the possibility of providing aid from his government for the injured and relatives of the victims.

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

The cause was not immediately known but Yeager said it was an anhydrous ammonia explosion.

Flames continued to flare, sparking fears of more explosions.

The Federal Aviation Administration declared a no-fly zone over the area around West, over fears another blast could bring down small aircraft.

Power and gas has been cut to some areas of the town as a precaution, Swanton added.

He said "air quality is a concern," adding that authorities were watching the wind patterns and "where the cloud may drift," and expect they will need to order further evacuations.

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 that experts "were able to see the ground motion that the explosion created," putting the magnitude of the shockwave at 2.1.

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.

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