At least 57 people have died in Pakistan after a bus collided with an oil tanker and burst into flames.

The head-on collision happened on a motorway near the Pakistani city of Karachi on Sunday morning. Reports suggest the tanker was speeding and on the wrong side of the road.

Authorities said the bus immediately caught fire following the crash.

Around 60 people were on board the bus as well as some passengers travelling on the roof, who were able to jump for their lives. Some estimate that as many as 57 people died after being trapped inside the burning vehicle.

Shoaib Siddiqui, Karachi commissioner, said: "All the bodies have been transferred to hospital. We estimated that around 44 bodies were brought to hospital and they are still in the mortuary."

"Four people who were injured, including two children and women, were allowed to go home after treatment. The bodies are charred and joined to each other badly."

Distraught relatives of the bus passengers gathered outside Karachi's Jinnah Hospital, where the bodies of the victims were taken.

One man, Abdul Hafeez, was overcome with emotion as he told the AP news agency that nine people from his family had died.

"My sister, her kids, two uncles and their families; a total of nine members of my family were on board and nobody survived," he said.

Pakistan has a record of frequent fatal traffic accidents due to poor roads, badly maintained vehicles and reckless driving.

The driver of the oil tanker fled the incident but it is not clear what caused the crash.

A total of 57 people, including 18 children, were killed last November when a bus collided with a coal truck in Sindh, near the city of Khairpur.