An explosion at a chocolate factory in Pennsylvania killed two people and left nine people missing, authorities said.

The explosion took place at the RM Palmer Co plant, the West Reading Borough Police Department said.

Chief of police, Wayne Holben, however, did not confirm the exact number of injured.

The explosion took place before 5pm on Friday, the local media reported.

Thick plume of black smoke rose from the factory after the explosion. One building was completely destroyed and a neighbouring building — that housed apartments —- suffered severe damage.

“It’s pretty levelled,” Samantha Kaag, mayor of West Reading Borough said of the explosion site.

“The building in the front, with the church and the apartments, the explosion was so big that it moved that building four feet forward.”

Mr Holden told the media that the police are investigating the cause of the blast.

The chocolate factory plant is situated about 60 miles northwest of Philadelphia.

Eight people were taken to the local hospital with injuries, it was reported.

According to the hospital spokesperson, two people we admitted in fairly stable condition and five were being treated. The spokesperson added that they would be released soon.

One patient has been, however, transferred to another facility. No further details regarding the condition of the injured were made available.

Meanwhile, residents in the neighbouring buildings were asked to leave the buildings to safety, according to Dean Murray, the borough manager of West Reading.

Ms Kaag also said people were asked to move back about a block in each direction from the site of the explosion but no evacuations were ordered.

PM Palmer chocolate plant employs 850 people at its West Reading headquarters, according to its website.

Liz Soto, who was near the explosion was quoted by Fox 29 as saying that she heard “a loud noise, like a roaring sound, then the house shook”.

Ms Soto has not yet heard from a friend who she said works inside the factory. “She went to work, she’s confirmed to have gone to work, but we don’t know anything about her.”