MULTAN, Pakistan -- A bus in Pakistan caught fire after hitting a van parked on the shoulder of an intercity highway in eastern Punjab province, killing at least 18 people and injuring 13 others, police and rescue officials said Sunday.

The accident occurred early Sunday near Pindi Bhattian, where the Islamabad-bound bus hit a van parked on the shoulder of the Lahore-Islamabad Motorway, senior police officer Fahad Ahmed said. The van was carrying fuel drums, which caused an inferno that engulfed the bus, Ahmed said.

There were more than 40 passengers on the bus, Ahmed said. Those who were rescued were badly burned, including several in critical condition. Other passengers were slightly injured with burns after escaping through the windows.

The drivers of both vehicles died, police said.

Such accidents happen frequently on Pakistan's highways, where safety standards are often ignored and traffic regulations violated. Fatigued drivers also fall asleep behind the wheel during long drives.