The incident took place when the bus, en route to Rajanpur from here, hit the trailer near Khanewal town in the country's Punjab province, according to a media report.
While 11 people died on the spot, 20 others were injured, the Express Tribune quoted police as saying. The bus driver was also killed, they said.
The injured, some of whom were said to be in critical condition, were shifted to a hospital in Multan, about 580 km from Islamabad.
Thick toxic smog has already disrupted normal life in parts of Punjab and Khyber Pakhtunkhwa, with flights cancelled and road accidents surging, it added.