The vehicle, which was heading to the capital of Islamabad, hit a van that was parked on the hard shoulder of the intercity motorway before bursting into flames on Sunday morning.

The drivers of both vehicles died, according to police.

The van was carrying three barrels of petrol, which caused an inferno that engulfed both the bus and van, senior police officer Fahad Ahmed said.

There were more than 40 passengers on the bus.

Several passengers are in a critical condition after being badly burned.

Others were forced to escape the bus through the windows, suffering from minor burns.

At first, the authorities said at least 18 people had been killed. That figure has now been changed to 20.

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Deadly collisions are common in Pakistan, where traffic rules are rarely followed and roads in many rural areas are in poor condition.

More than 40 people died in January after a bus fell into a ravine and caught fire in the southern Pakistani province of Balochistan.