Karachi Commissioner Shoaib Siddiqui said the accident took place on the Super Highway when the bus going from Karachi to Shikarpur collided with the oil tanker early today.
"It was the mistake on part of drivers of both vehicles as they were over speeding and the passenger bus was overloaded with some passengers even sitting on the roof," Siddiqui told PTI.
There were around 60 to 70 passengers in the bus and those sitting in the back leapt out of the windows to save their lives after the bus caught fire.
At least 40 passengers out of the 59, including women and children, were burnt to death. A family of eight were also among those who were killed.
Also Read
One survivor told Geo News channel that the passenger bus was overcrowded and the driver was also over speeding.
Television channels showed both vehicles on fire as rescue workers and fire engines went about their work.
Road in the Sindh province are notorious for being poorly maintained while lax checking by the highway police also emboldens drivers to over speed during night time.
Last November, in a similar accident around 57 people including women and children were killed when a passenger bus collided with a goods truck loaded with coal near Khairpur on the Super Highway.
In March, 43 people were killed when an oil tanker crashed into a trailer tractor on the highway.
Bad roads, driver's negligence, over speeding and overloading are the main reasons behind major accidents in Pakistan.