"Three buses were racing. One of them rammed into an oil tanker," police official Imran Shaukat told Reuters, describing the incident before dawn on Tuesday on a motorway near the central city of Multan.

The crash triggered a fire that engulfed both the tanker and the bus, he said. The oil tanker was beyond identification to determine what company it belongs to, he said.

A spokesman for the motorway police said the tanker's driver fled the scene.

Officials from a state-run rescue service said the bus was bound for the southern port city of Karachi.

The rescue service identified the operator as the Daewoo bus company and said at least 20 passengers had died, some of them burnt completely.

Six passengers survived, an official of the rescue service told Dunya TV.

"The fire was raging when we got here," he said, adding that most of the passengers were caught by the fire in their sleep.

Deadly road accidents are common in Pakistan, mainly due to speeding and poor road infrastructure.

Australian Associated Press