The death toll from an explosion at a chocolate factory in the US state of Pennsylvania rose to three Saturday with four people missing, local officials said, as rescuers raced against the clock to find survivors.

Crews were still removing debris and rubble following the Friday afternoon explosion at the RM Palmer Company plant in the West Reading borough of Pennsylvania.

However, "due to the violence of the explosion and the amount of time that has passed, the chance of finding survivors is decreasing rapidly," West Reading Fire Chief Chad Mayor told a press conference Saturday.

TV footage showed a fire blazing in the debris after the explosion, with firefighters trying to extinguish it.

"(The factory) is pretty leveled, unfortunately. There's not much they can salvage from the building," West Reading Mayor Samantha Kaag said in a televised press conference shortly after the blast.

"In the front, with the church and the apartments, the explosion was so big it moved that building four feet forward," she added.

West Reading Chief of Police Wayne Holben told reporters at the press conference Saturday that "a third body was located at the RM Palmer site and has been confirmed deceased."

"Crews are continuing their diligent efforts in sifting through the debris in order to locate any additional individuals," Holben said.

Fire Chief Mayor added that "currently we have four individuals that are unaccounted for."

The cause of the blast was still under investigation, authorities said.

According to its website, RM Palmer has been producing seasonal chocolate novelties -- like Easter bunnies and heart-shaped Valentine's Day sweets -- since 1948.