At least 57 people including women and children were killed when their bus crashed into an oil tanker, igniting a fierce blaze in southern Pakistan, officials say.

Authorities fear the death toll could rise, with most of the bodies burnt beyond recognition. Initial reports on Sunday said the oil tanker was travelling in the wrong direction along the dilapidated stretch of road, police said.

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“We have received more than 57 dead bodies but the death toll may rise as most of them are completely burnt and stuck to each other,” Doctor Semi Jamali at Karachi’s Jinnah hospital said.

Jamali said the bodies of at least six children were stuck to women who may have been their mothers, adding it was impossible to separate the remains.

Families of the victims mourns their loss. Photo: AAP

“They are beyond recognition, they can only be identified by DNA test,” she said.

The overloaded bus, carrying more than 60 passengers, was en route to the town of Shikarpur from the southern port city of Karachi when the collision occurred.

Television channels showed live footage from the fiery crash site where rescue workers were busily evacuating dead bodies and any injured.

Earlier, senior police official Rao Muhammad Anwaar said the bus “hit the oil tanker, which according to initial reports was coming in the wrong direction” and caught fire.

Another senior police official, Aamir Shiekh, said an investigation has been launched but it appeared the poor condition of the single track road also contributed to the cause of the accident.

“We are trying to ascertain if the driver of the oil tanker was solely at fault or whether the bus driver also showed negligence,” Anwaar said.

A few passengers escaped unhurt after they jumped out of the bus windows, police official Muhammad Jan said.

It was the second major fatal crash in Sindh province in less than three months.