Hundreds of people rushed to the truck depot in Hyderabad city to search through the rubble of destroyed shops for dead and wounded, local television footage showed.
Authorities ruled out terrorism and determined the blast occurred from a pressure buildup, said Mohammad Ali Baloch, the senior police official in Hyderabad.
The truck was carrying 7,925 gallons (30,000 liters) of “thinner,” said Babar Khattak, the police chief in Sindh province where Hyderabad is located. He was not more specific about the chemical being transported.
"We heard a huge explosion after which a thick blanket of smoke covered the whole area, nothing was visible," laborer Mohammad Hussain said. "When the smoke cleared I saw many of our laborers had died and vehicles destroyed." Ali Mohammad Baluch, a senior police officer in the city, told AFP that a total of 18 people had been killed and 40 injured.
"It was an accidental blast. It was not an act of terrorism," provincial government spokesman Jamil Soomro said.
Witnesses said the huge explosion destroyed the tanker, sending large fragments flying into laborers and vehicles in the area. Power supplies were also cut as some flying debris hit electrical wires, residents said.
Several nearby shops and old buildings were toppled due to the force of the explosion, trapping some residents.
"The severe explosion hit shops in the nearby market. Several shops collapsed, we are removing the debris," said provincial police chief Salahuddin Babar Khattak, who confirmed the death toll and the cause of the explosion.
Hyderabad police chief Fayyaz Leghari said the tanker exploded when it was parked at a terminal in a commercial suburb of Hyderabad, which has an estimated population of three million.
"It looks like an accidental blast, we cannot say it is an act of terrorism," he added.
Pakistan suffers from chronic insecurity largely connected to the country's alliance with the United States in its "war against terror."
On Monday, Taleban fighters ambushed a Pakistan army convoy as it passed through a village in the Bajur tribal area, killing four soldiers, said Mohammad Jamil Khan, a senior government official in Bajur.
The military retaliated and killed three militants, he said. The attack occurred in Mamound village, some 15 miles north of Khar, the main town in Bajur.
A campaign of suicide and bomb attacks blamed on Al-Qaeda, the Taleban and other extremist groups has killed more than 3,400 people in less than three years across the nuclear-armed country of 167 million.
