The bus carrying more than 70 people from the minority Christian community was coming to Rawalpindi from Sialkot when it caught fire near Jhelum, about 100 kilometres south of the capital Islamabad, local police official Najmul Hasnain said.
The vehicle's engine stopped and the driver was trying to convert it from fuel gas to diesel when it burst into flames, he added.
"Twelve people were burnt to death, they included six children, four women and two men," he said.
Pakistan has one of the world's worst records for fatal traffic accidents, blamed on poor roads, badly maintained vehicles and reckless driving.
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