An explosion at a chocolate factory in Pennsylvania on Friday evening killed two people and left several others missing as investigators begin to determine a cause.

West Reading Borough Police Department Chief of Police Wayne Holben confirmed there were two fatalities, nine people missing and several others injured by the explosion at the R.M. Palmer Co. plant in West Reading.

The explosion at 4:57 p.m. sent a plume of black smoke into the air and resulted in the destruction of a building at the facility and damage to a neighbouring building. The cause of the explosion remains under investigation, Holden said during a press conference.

There was no further danger, but Holden directed residents to avoid the area of the factory about 96 kilometres northwest of Philadelphia.

Tower Health spokeswoman Jessica Bezler said eight people were taken to Reading Hospital on Friday evening.

Two were admitted in fair condition and five were being treated and would be released, she said in an email to The Associated Press. She said one patient was transferred to another facility, but provided no other details.

Berks County fire crews remained on the scene on Friday night. (AP)