At least 59 people have been killed after an oil tanker apparently speeding in the wrong direction crashed into a passenger bus in southern Pakistan, igniting a fierce blaze, medical sources have said.

The bus was en route to the town of Shikarpur from the southern port city of Karachi when the collision occurred along a stretch of dilapidated road early on Sunday.

Senior police official Rao Muhammad Anwaar told the AFP news agency that the bus caught fire after the accident. The victims included women and children, some of whom were charred beyond recognition, Anwaar added.

Semi Jamali, a doctor at Karachi's Jinnah hospital, told the AFP news agency: "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."

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

"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.

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

Prime Minister Nawaz Sharif deplored the loss of lives and extended condolences to the families of those who lost their loved ones. He directed for provision of best medical facilities to the injured and asked concerned authorities to take steps to improve road safety.

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

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

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, 450km north of Karachi, the capital of southern Sindh province.

An average of 4,500 people have been killed in road accidents each year since 2011, according to the Pakistan Bureau of Statistics.