At least 19 people were killed and at least 20 were injured on Tuesday when a double-decker passenger bus caught fire after a vehicle collision on one of Thailand's busiest highways, officials said.

Police said an 18-wheeled truck crashed into the Bangkok-bound bus on Mitraphab highway in the central province of Saraburi early Tuesday.

Officials said most passengers were stuck in the bus when it caught fire, including the bus drivers. Authorities are still investigating the cause of the crash and working to identify the dead bodies. It wasn't known if the truck driver was among the dead.

Another woman was injured when a pickup truck crashed into the collided pile of vehicles.

Saraburi is 90 kilometers (60 miles) north of Bangkok.