The battered truck was left in the middle of a narrow road on the border of Helmand and Kandhar provinces for several days after insurgents opened fire on it.
Police considered the area too dangerous to enter, the officials said.
Before sunrise Friday, the bus smashed into the truck and both vehicles burst into flames, badly burning many of the bus passengers, said Abdul Razaq, the provincial police chief of Kandahar.
Razaq said eight passengers were injured Omar Zawak, the governor's spokesman in Helmand province, put the injury total at 11. Both said the casualties included men, women and children.
The bus began its journey in the capital of Helmand province and was scheduled to stop in Kandahar city, then travel north to Kabul, the afghan capital, Razaq said.
GEO TV