The incident took place near Theri bypass in Khairpur area of Sindh province and police said that early morning thick fog and careless driving were the main reason for it.
The death toll after the deadly accident rose to 20 from 17 after three of the seven injured succumbed to their wounds, Geo News reported.
Police and rescue officials said the death toll could rise as the some injured were in precarious condition.
The victims were shifted to a nearby hospital, where two injured passengers are said to be in critical conditions.
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Road accidents are common in Pakistan. Most of them are caused by careless driving, bad roads and faulty vehicles.
At least 59 people were killed while 18 others were injured when a passenger bus collided head-on with a truck on the same Theri bypass in 2014.
On November 9, a speeding bus carrying over 100 passengers fell into a ravine killing at least 27 people and injuring 69 others in Punjab province.