An explosion at a chocolate factory in Pennsylvania has killed two people with five others missing as rescue workers search through debris for survivors, US emergency services say.

The blast occurred on Friday at the RM Palmer Company plant in West Reading, a town about 95km northwest of Philadelphia, the Pennsylvania Emergency Management Agency (PEMA) said.

West Reading officials said on Saturday they could confirm only two fatalities.

The PEMA had earlier said there were five fatalities, citing county emergency management officials, but after an update from the county also indicated that two had died and five were missing.

PEMA said an investigation would be conducted to determine the cause of the explosion, and that an initial incident report included reference to a gas leak.

Seven other people were transported to local hospitals with injuries, PEMA added.

Images on social media of the explosion’s aftermath showed a vast pile of debris and massive flames letting off large plumes of thick black smoke, in a downtown area surrounded by other buildings.

The RM Palmer Company was founded in 1948.

It employs 850 people and creates 500 unique chocolate and candy products, according to its website.

The company did not immediately respond to a request for comment.

— AAP with AP