At least 57 people including women and children died when their bus crashed into an oil tanker, igniting a fierce blaze in southern Pakistan early Sunday, officials said.
Authorities fear the death toll could rise, with most bodies burnt beyond recognition. Initial reports said the oil tanker was travelling in the wrong direction along the dilapidated stretch of road.
"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 told AFP.
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.
"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.
Pakistan has an appalling record of fatal traffic accidents due to poor roads, badly-maintained vehicles and reckless driving.
At least 57 people, including 18 children, were killed in November last year when a bus collided with a goods truck loaded with coal near Khairpur town, 450 kilometres (300 miles) north of Karachi, the capital of southern Sindh province.
Police blamed the poor condition of the road and the lack of warning signs to alert drivers to the danger, and said they would investigate the government department responsible- the first time such an investigation has been launched in the country.
Last April a bus smashed into a tractor-trailer in a high-speed collision in Sindh, killing 42 people, while in March a horrific crash between two buses and a petrol tanker left 35 dead, with many burned alive when the fuel ignited.
The recovery equipment available to Pakistani emergency services is often basic, and when crashes happen away from major towns, rescue efforts can take some time, often reducing injured passengers' chances of survival.
The mountainous areas of Kashmir and the north, where drivers career around narrow hairpin bends over deep ravines with scant regard for safety, are particularly prone to accidents, while the condition of roads in the south can also be dangerous.