Texas Waco fertiliser plant blast search for survivors
Emergency services are still searching for survivors after a blast at a US fertiliser plant that police say killed up to 15 people.
More than 160 people were injured and dozens of buildings destroyed in the town of West, near Waco in Texas.
The town's Mayor Tommy Muska said he believed at least 14 people were dead, including four firefighters who attended the original fire.
The subsequent explosion produced a tremor similar to a small earthquake.
It may have been caused by the chemical ammonia, according to emergency services officials.
There is no indication that the blast and a fire which preceded it were anything other than industrial accidents, police say.
However, the site is being treated as a crime scene and the death toll could rise, officials warn. There has been no confirmation of the number of people killed yet.
Texas state Governor Rick Perry said he was declaring the area a disaster and would request an emergency declaration for federal aid from US President Barack Obama, who has apparently offered any help needed.
Air quality and gas pipelines are being monitored for safety, he said. Gas supplies have also been disconnected until any risk has passed.
A search-and-rescue operation is continuing, the Texas Department of Public Safety said late on Thursday.
The situation remains volatile because of an extremely dangerous substance, ammonium nitrate, found at the scene, said an official from the county sheriff's office.
Experts from federal and state environmental agencies were going in to determine how dangerous it was for the emergency services, the official said.
Earlier, police said the plant was storing tanks of anhydrous ammonia, a less flammable fertiliser.
"Anhydrous ammonia will burn but it is unusual for it to detonate, whereas ammonium nitrate has a track record of blowing up," explained Prof Andrea Sella, a chemist at University College London.
"What we do know is that if you formulate it in the right way, it actually forms kind of the bedrock of the mining industry - it's a hugely used commercial explosive," he told the BBC World Service.
Officials refused to confirm the exact number of people killed. For security reasons, the Bureau of Alcohol, Tobacco and Firearms is requesting no-one enter the affected area.
The explosion devastated the West Fertilizer Company, about 20 miles (32km) north of Waco, in central Texas.
Dozens of homes were levelled and other buildings - including a school and nursing home a few hundred metres from the plant - were badly damaged.
Jason Shelton, 33, who lives less than a mile (1.6 km) from the plant, told the Reuters news agency he felt the concussion from the blast as he stood on his front porch.
"My windows started rattling and my kids screaming," Mr Shelton said. "The screen door hit me in the forehead... and all the screens blew off my windows."
More than 130 people had already been evacuated from the nursing home by the time of the explosion because the fire was recognised as a risk.
It was not immediately clear how many of them were hurt, a public safety department spokesman told a news conference.
A husband and wife who entered the nursing home before the emergency services arrived found residents in wheelchairs trapped in their rooms, the Associated Press (AP) reported.
"They had Sheetrock [plasterboard] that was on top of them. You had to remove that," William Burch told AP. He described hallways filled with water and electrical wires hanging from the ceilings.
Governor Perry said the school would remain closed until administrators felt it was the right time to re-open.
A small amount of looting had been reported, police said. The state attorney general also warned against price-gouging, saying that anyone who tried to profit from the explosion by putting up prices on essential items would face a lawsuit.
US President Barack Obama said his administration was in close contact with emergency services at the scene.
"West is a town that many Texans hold near and dear to their hearts, and as residents continue to respond to this tragedy, they will have the support of the American people," he said in a statement.