At least 21 people including 19 children were killed in Pakistan on Wednesday when a school van smashed head-on into a heavy truck, officials said.

The van collided with the loaded dump-truck near the town of Nawabshah, around 270 kilometres (170 miles) north of Karachi in the southern province of Sindh.

The children, who were students of the Bright Future School from the nearby town of Daulatpur, had come to Nawabshah for a school trip.

Their van was on its way back to Daulatpur when the accident happened.

Doctor Hashim Langa, chief of the government-run Civil Hospital in Nawabshah, said the hospital had received 21 dead bodies out of which 19 were children.

"Their ages range from five or six to 16," he told AFP.

Apart from the dead, some 15 to 20 injured were also brought to the hospital, seven of whom were in intensive care and neurological wards, he added.

Police said that they were trying to locate the driver of the truck who had fled the scene.

Senior police officer Asif Ali Pechuho told AFP that the school van had been so badly wrecked that rescue workers had to use tools to extract bodies.

The driver of the school van was among the dead, he added.

Pakistan has one of the world's worst records for fatal traffic accidents, blamed on poor roads, badly maintained vehicles and reckless driving.