At least 40 people died when a bus plunged off a bridge in southwestern Pakistan and burst into flames, a government official said on Sunday.

"The dead bodies...are beyond recognition," Hamza Anjum, a senior official of Lasbela district in Balochistan province, said at the accident site.

Anjum said three survivors had been rescued and the bus was reportedly carrying 48 passengers when it hit a pillar on the bridge and careened off course earlier on Sunday.

It had been travelling overnight between Balochistan's provincial capital of Quetta and the southern port city of Karachi.

"It is feared that the driver may have fallen asleep," Anjum said, also mentioning the possibility the driver had been speeding during the long-distance trip.

"We will investigate the causes of the accident," he said, adding that DNA tests would be used to determine the identity of the remains which had been "badly mutilated".

Ramshackle highways, lax safety measures and reckless driving contribute to Pakistan's dire road safety record.

Passenger buses are frequently crammed to capacity and seatbelts are not commonly worn, meaning high death tolls from single vehicle accidents are common.

In November, 20 people, including 11 children, were killed when a minibus crashed into a deep and water-logged ditch in southern Pakistan.

And last August, 20 people also perished on the outskirts of the city of Multan when a bus collided with an oil tanker.

According to World Health Organisation estimates, more than 27,000 people were killed on Pakistan's roads in 2018.