The oil tanker coming from Karachi and headed to Lahore overturned early this morning on the national highway at the Ahmedpur Sharqia area of the Bahawalpur district, some 400 km from Lahore, after a tyre burst and the driver lost control.
The fire was apparently caused by someone who lit a cigarette after people from nearby localities gathered on the highway to collect petrol, officials said.
"This is a tragedy of its kind in the country's history that left everyone in grief in Pakistan. My thoughts are with the victims' families," Sharif said before leaving for Pakistan.
Rescue officials said the blaze from the oil spill engulfed scores of residents, killing 151 people and injuring more than 140 others.
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"Some 151 people including women and children lost their lives in this tragedy. The condition of more than 50 injured is critical and they are being shifted to Lahore and Faisalabad cities to save their lives," Rescue official Jam Sajjad told PTI, adding that the death toll could rise further.
District Coordination Officer (DCO) Bahwalpur Rana Salim Afzal termed it a "huge tragedy" in the history of Pakistan.
"At least 123 people were burnt alive and died on the spot while the injured were shifted to the district headquarters hospital and Victoria Hospital in Bahawalpur where the condition of most of them is critical," Afzal said, adding that about 50,000 litres of fuel spilled from the oil tanker.
Muhammad Hanif, 40, who suffered burns, told reporters at the Victoria Hospital that he was present at his house when his cousin called him informing that the village people were rushing to the highway to collect "free oil".
"Suddenly the tanker burst and the people gathered near it were burnt alive. Rashid and I were a little away from the tanker therefore we are alive," Hanif said.
He said it was "greed" of the villagers which took them to the "valley of death".
The leaking fuel was even flowing along the highway in the fields. People were seen sitting near the fields collecting fuel in cans and bottles.
"The people from nearby villages had also gathered there. The motorway police personnel who also reached the accident site asked them to leave the place but they started collecting petrol. Suddenly the tanker exploded and within seconds the fire erupted giving no chance to the people present there to leave the place," Regional Police Officer Bahawalpur Raja Rifat said.
"It was a scene of the Day of Judgement. Before my eyes a large number of people fell prey to a ball of fire. I was sitting in my car a little away from the oil tanker thinking should I join these people and fill my car fuel tank with free oil when suddenly the tanker exploded and all the people close to it trapped in fire and burnt alive within no time," an eyewitness named Shahzad Hasan said.
Hasan said he along with some other people shifted three injured to the Victoria Hospital.
The Punjab government and army helicopters shifted the critically burnt people to Multan, Lahore and Faislabad for better health facilities.
According to a government official, at least 75 bikes and about a dozen cars were also burnt at the site.
Punjab Chief Minister Shehbaz Sharif announced Rs 2 million each for victims and Rs 1 million each for the injured. He also reached Bahawalpur to ensure best medical treatment for the injured.
Prime Minister Nawaz Sharif, President Mamnoon Hussain, Pakistan Tehreek-e-Insaf chairman Imran Khan and Pakistan Peoples Party chairman Bilawal Bhutto condoled the tragedy.
Army helicopters have been deployed in the rescue operations.
The tragedy came a day ahead of Eid ul-Fitr celebrations in the country, marking the end of the holy fasting month of Ramzan.
The oil tanker tragedy comes about two years after 62 people, including women and children, were killed in a fire after a coach collided with an oil tanker travelling on the wrong side of the road, on the outskirts of Karachi.