Fifty-eight people have been killed in a head-on collision between a passenger bus and a truck on a highway in southern Pakistan, police said.

The crash ignited a fuel fire and a rescuer later told how he carried out a survivor, a four-year-old girl, from the burning bus.

The bus, carrying about 70 people, had left the Swat Valley and was en route to the southern port city of Karachi when the accident happened near Khairpur district in Sindh province, said police official Ghulam Jhokhio.

He said the bus quickly caught fire after its fuel tank exploded.

The fatalities included 21 women and 19 children, all under the age of 14, said local hospital official Jafar Soomro, warning that the death toll was likely to rise. Fifteen people were injured and in hospital, several of them in a critical condition, he added.

Initially, Mr Jhokhio said the crash might have happened because of heavy fog.

But later deputy chief of highway police AD Khawaja said the bus was speeding on a part of the highway under construction and that the driver's carelessness caused the collision.

"The speeding bus was overcrowded and the driver was careless," he said, adding that, earlier in the day, traffic police had stopped the bus and fined it for carrying too many passengers.

Private Pakistani TV channels broadcast live footage from the scene, showing rescue workers carrying the victims and policemen clearing the road.

Rescue officer Mohammad Ata described the inferno to Dunya TV as he held a little girl in his arms, and recounted how he pulled her out of the burning bus.

"She was sitting all calm in a seat when I got into the bus on fire," he said.

Fatal accidents are common on roads across Pakistan due to bad road infrastructure and rampant disregard of traffic laws. More than 9,000 road accidents are reported to the police every year, killing on average around 5,000 people, according to the Pakistan Bureau of Statistics.

A Sindh provincial minister, Siraj Durrrani, decried the tragedy and said the government badly needs to improve the infrastructure to avoid such horrific accidents.