More than 118 people were injured in the explosion in the eastern province of Punjab, which came as people gathered to collect leaking fuel after the tanker overturned on Sunday, government officials and rescue workers said.
The disaster was one of several deadly events to mar the run-up to Monday's Eid al-Fitr holiday marking the end of the Muslim fasting month of Ramadan. At least 65 people were killed in bombings in two cities on Friday claimed by Islamist terrorists including the Pakistani Taliban and Lashkar-e-Jhangvi.
A burst tyre apparently caused the speeding fuel tanker to overturn on a sharp bend on a highway on the outskirts of the city of Bahawalpur, provincial spokesman Malik Muhammed Ahmed Khan said.
It was not clear what triggered the fireball but on Sunday, a spokesman for rescue services said someone was believed to have lit a cigarette.
"Rehabilitation and treatment of the victims is our top priority," Khan said.
Hospitals are also arranging DNA testing to identify the many bodies that were charred beyond recognition. At least 20 children are among the dead.
Many of the burn victims have been moved the larger cities of Karachi and Lahore. Governments hospital are often-underequipped and the ones in Bahawalpur and nearby Multan were set up only to handle small numbers, Khan said.
He said a separate investigation was underway to see why police did not disperse the crowds of people who gathered to collect fuel.
"Police did try but we are looking into what went wrong," he said.
Prime Minister Sharif visited the injured in Bahawalpur's Victoria Hospital on Sunday after flying home from a trip to London, his office said.
Television footage showed the prime minister standing by the bedsides of victims.
Sharif had been in London to attend his grandson's graduation ceremony, his daughter Maryam said in a tweet.