A head-on collision between a mini-bus and a truck in southern Pakistan on Monday killed at least 20 people and left three others injured, police said.

According to Associated Press, police officer Rab Nawaz said the fully loaded truck crashed into the passenger van on a cross-country highway near the town of Khairpur, south of the capital, Islamabad. 

Police initially said 17 were killed but three of the injured later died at hospital.

AP reported that Nawaz said the others injured were also in critical condition and that the death toll could increase further. 

The police said the passengers included women and children. Both drivers were killed.

The initial investigation showed that speeding amid poor visibility in the morning fog had caused the tragedy, Nawaz added. He said the recovery of the victims’ bodies was made difficult as the heavy truck had overturned after the collision, landing on top of the mini-bus, AP reported.

Road accidents are common in Pakistan because of poor road conditions and disregard for safety standards and traffic rules by drivers.