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.
Pakistan has been wracked by terrorist attacks over the past few years, prompting the army to launch a series of offensives against the Pakistani Taliban along the country's northwest border with Afghanistan.
Taliban fighters ambushed an army convoy Monday 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 (25 kilometers) north of Khar, the main town in Bajur.
The military has twice declared victory in Bajur, but attacks blamed on the militants continue.
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Associated Press writer Habib Khan contributed to this report from Khar.
