At least 18 passengers, including women and children, were killed and 16 injured in a fatal accident on a motorway in Punjab province of Pakistan on Sunday morning, police said.

The bus en route to Islamabad from Karachi carried about 40 passengers when it collided with a pick up van at Pindi Bhattian section of Faisalabad motorway, some 140 kilometres from Lahore, around 4:30 am, the police said.

"On the Pindi Bhatian section of the motorway, the bus hit a static van which was carrying a fuel tank. The bus hit it from the rear and both vehicles caught fire immediately, killing at least 18 passengers," Inspector General (IG) Motorway Police Sultan Khawaja said.

Another 16 persons with burn injuries were shifted to nearby hospitals.

The death toll may rise further as the condition of some of the injured is said to be critical, the IG said.

The IG further said the passengers who managed to jump off the bus survived.

Others did not get a chance to get off as fire engulfed both the vehicles, he said, adding that drivers of both the vehicles also died.

IG Khawaja also said the cause of the accident was yet to be ascertained "whether the bus driver fell asleep as the time of accident or the crash took place because of over speeding".

He said both the vehicles would not have caught fire had the van not been carrying a fuel tank.

He added that the identity of those who lost their lives would be ascertained through DNA tests after which the bodies would be handed over to their families.

Punjab caretaker chief minister Mohsin Naqvi expressed grief over the loss of lives in the accident and directed the authorities concerned to provide best treatment to the injured.

Road accidents which are common across Pakistan claim numerous lives on a daily basis.

Five people, including a woman and two minors, were killed and 20 others injured when a bus overturned in the Fazilpur area of Punjab's Rajanpur district back in July.

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