Rescue teams are still searching the charred rubble of homes for survivors of an explosion at a fertiliser plant that killed as many as 15 people, injured more than 160 and destroyed dozens of buildings.

Three or four volunteer firefighters were among the missing after the blast near the Texas town of West – 20 miles north of Waco – on Wednesday night.

Fire crews had been fighting a blaze at the West Fertilizer Company's factory for about 20 minutes before an explosion rocked the town, which has a population of 2700 people.

It produced a tremor equivalent to a small earthquake.

The plant had tanks of volatile anhydrous ammonia, including what initial reports said was a tanker-sized container like those carried on freight trains.

However, Sergeant William Patrick Swanton, of the Waco police department, told a press conference that the immediate threat from fumes appeared to have abated.

The blast destroyed 60 to 80 houses, officials said. It reduced a 50-unit apartment complex to what one local official called "a skeleton standing up" and left a horrific landscape of burned-out buildings and blackened rubble.

"It looks like a war zone with all the debris," McLennan County Sheriff Parnell McNamara said.

Amid such devastation, the death toll remained a rough estimate of five to 15 people, Mr Swanton added.

Three hospitals in Waco and Dallas treated more than 160 people injured in the blast, but police said that total was likely to rise.

Mr Swanton said many people were still missing, including three or four volunteer firefighters.

They had been battling the blaze at the fertiliser plant, and evacuating nearby houses and a nursing home after concerns about possible dangerous fumes, when the explosion occurred.

Another firefighter, who is also a law enforcement official, was found badly injured. He is in a critical condition in hospital.

Texas Public Safety Department spokesman D L Wilson said half the town had been evacuated. Officials said 133 people had been removed from the nursing home.

Last night emergency crews were moving from house to house in a search-and-rescue operation.

Mr Swanton said: "That's good news. It means they are probably still getting injured people. They have not got to the point of no return where they don't think there is anybody still alive."

Eyewitness Kevin Smith said he had just climbed the stairs to the second floor of his home when he felt the blast.

He said: "The house exploded. It was just a bright flash and a roar, I thought it was lightning striking the house. I felt myself flying through the air.

"It took a second or two to realise the roof had caved in on me so I knew it wasn't lightning."

The cause of the fire remained unknown and the area is being treated as a crime scene, which is standard procedure according to police.

The incident is now being investigated by several US agencies, including the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

President Barack Obama, who attended a memorial service for those killed in the Boston Marathon bombing yesterday, offered support and prayers to the victims in Texas.

Ground motion from the blast registered as a magnitude 2.1 seismic tremor and created a jolt felt 80 miles away in Dallas, the US Geological Survey reported.

The explosion came two days before the 20th anniversary of a fire in Waco that engulfed a compound inhabited by David Koresh and his followers in the Branch Davidian sect, ending a siege by federal agents. About 82 members of the sect and four federal agents died at Waco.