Sixteen schoolchildren and a teacher were burned to death in eastern Pakistan when a faulty gas cylinder exploded on the bus taking them to school, police said.
A police officer said five children were also injured, three of whom were listed in critical condition, in the fire.
The blaze occurred on the outskirts of the city of Gujrat, about 200km northwest of Islamabad.
Tahir Naveed, a medical superintendent at Gujrat district hospital, said it had received seven people who were injured in the accident. The discrepancy could not be immediately explained.
The dead and injured children were aged between 5 and 13, said Naveed.
Earlier, police had blamed an exploding natural gas cylinder for the incident. The bus was powered with both types of fuel.
TV footage showed the charred minibus and relatives of the victims standing outside a hospital morgue.
A schoolgirl told Pakistan's Geo news channel that the driver escaped when the vehicle went up in flames. Authorities said they were trying to track him down and arrest him.