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 20 people and injuring 11 others, police and rescue officials said Sunday.

Read more: Pakistan arrests 129 Muslims after mob attacks on churches and homes of minority Christians

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.

Read more: Muslim mobs attack churches in eastern Pakistan after accusing Christians of desecrating the Quran

The drivers of both vehicles died, police said.

Mohammad Jawed, a medical officer at a hospital in nearby Faisalabad district, said two of the seriously burned injured died in the hospital, taking the death toll to 20 with 11 hospitalized.

Read more: Pakistan’s caretaker premier is to be sworn in as people celebrate Independence Day

Ehsan Zafar, a spokesman for the motorway police, said the fire completely gutted the bus and only the steel frame remained. He said the fire fighting squad reached the scene in minutes but by that time fire had engulfed both vehicles. He said initial investigation suggests that the accident was due to negligence on the part of the bus driver, who hit the parked vehicle from behind.

Imdad Ali, who was on the bus, said most of the passengers were sleeping when the crash happened.

“The jerk due to the accident woke everybody but the fire gave no time to passengers to escape. Flames quickly covered the bus like an envelope and all women, children and men were screaming,” Ali said. He said he succeeded in breaking a window but hesitated over climbing through the sharp broken glass.

“Suddenly someone pushed me and I fell outside, thank God, and thanks to that man whosoever he was that I am alive.”

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.