WEST, Texas (AFP) - Rescuers in Texas sifting through the rubble of homes destroyed when a fertiliser factory exploded earlier this week have recovered 12 bodies but 60 people remain missing, officials said on Friday.

Approximately 200 other people were injured in Wednesday's blast in this small town, Texas Department of Public Safety spokesman Jason Reyes told a news conference.

Investigators are treating the blast site as a crime scene, although authorities say the blast was likely caused by a fire at the West Fertilizer Company.

Fifty homes were destroyed, while teams have searched 150 homes and have another 25 to go.

"We are still in a search and rescue mode," Mr Reyes said.

Until now, officials had refused to release a firm death toll, saying only that it was between five and 15.

Speaking at the same press conference, Senator John Cornyn said that 60 people were still unaccounted for in the "terrible tragedy."

Streets were largely deserted Friday in the town with a population of about 2,800, 129km south of Dallas.

Texas Governor Rick Perry was scheduled to address the media at 5.00pm (6am Singapore time).

The blast came with the entire country already on edge after the attacks on the Boston Marathon that left three dead and more than 170 wounded on Monday.

A pharmacist who lived near the fertiliser plant told AFP his home had sustained major damage in the blast.

"I saw the house, it's in bad shape. The door is blown in, windows are blown in. All of the ceiling has collapsed onto the floor. I think that the roof is still intact but it was dark and I didn't get a real good look," said Mr Michael Sulak, a lifelong resident of West.

Mr Patrick Swanton, a local police sergeant, said the search was "very slow, methodical, tedious".

"They are having to do a lot of shoring up of homes to be able to make sure they are safe to get into," he added.

"They will take the necessary time to investigate this thoroughly to determine what happened and make sure that if in fact there was any criminal wrongdoing, that they find that and be able to figure that out."

The West Fertilizer Company was fined by US regulators in 2012 over its transport of hazardous materials, documents showed.

Last year, the company was fined US$10,100 (S$12,500) by the US Pipelines and Hazardous Materials Safety Administration for mislabeled cargo tanks and "failing to develop and adhere to a security plan" for transporting a large quantity of anhydrous ammonia, according to a copy of the citation.

The company reached a settlement with US regulators in which it paid a US$5,250 fine.

The violations concerned the transport of anhydrous ammonia - which lacks water - and not its storage at the factory itself, which exploded nearly an hour after a fire broke out on Wednesday evening, according to local officials.

"We are hoping for the best but we are preparing for the worst," said Mr Swanton.