It was the second crash involving major loss of life in Sindh province in less than three months.
AFP adds: Authorities fear the death toll could rise, since all the bodies were burnt beyond recognition, and are planning DNA tests for identification.
“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 adhering 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 tests,” 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.
Initial reports indicated the oil tanker was on the wrong side of the road along a dilapidated stretch of highway, police said.
“The bus and all passengers were so badly burnt that we have to carry out DNA tests for identification,” Sindh province information minister Sharjeel Memon told reporters.
Gul Hassan, a resident of Karachi, said he lost nine relatives including the 80-year-old head of the family and a two-year-old child.
The dead also included two women and another child, Hassan said.
“They were travelling in the same bus. I cannot recognise any of them, all the bodies are completely burnt,” he said.
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.
“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.
Another senior police official, Aamir Shiekh, said an investigation had been launched but it appeared the poor condition of the single-track road also contributed to the accident.
A few passengers escaped unhurt after they managed to jump out of the bus windows.
Pakistan has an appalling record for fatal traffic accidents due to poor roads, badly maintained vehicles and reckless driving.
A total of 57 people, including 18 children, were killed last November when a bus collided with a coal truck in Sindh near the town of Khairpur 450 kilometres (300 miles) north of Karachi.
Police blamed the poor road condition and the lack of warning signs and said they would investigate the government department responsible — the first time such an investigation had been launched in the country.
In April last year a bus smashed into a tractor-trailer in a high-speed collision in Sindh, killing 42 people.
In March 2014 a horrific crash between two buses and a petrol tanker left 35 dead, with many burned alive when the fuel ignited.
Rescue equipment available to Pakistani emergency services is often basic. When crashes occur far from major towns, rescue efforts can take time and injured passengers have less hope of survival.
The mountainous areas of Kashmir and the north, where drivers career around narrow hairpin bends above deep ravines with scant regard for safety, are particularly accident-prone.