At least 18 people were killed after a bus crashed and caught on fire in Pakistan's eastern Punjab province on Sunday, authorities said.

The bus collided with another vehicle in the early hours of the morning while carrying 33 passengers from the southern port city of Karachi to the capital, Islamabad.

"The unfortunate bus ... collided with a mini-truck loaded with three small tanks of petrol at around 4.30 am," said Fahad Ahmad, the police chief of Punjab's Hafizabad district.

"The bus and the truck caught fire immediately, the result of which 18 people including the mini-truck driver died on the spot."

He said another 16 people with serious injuries had been transferred to hospital.

Fatal road accidents are common in Pakistan, where traffic rules are rarely followed and roads in many rural areas are in poor condition.

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