At least 18 people burned to death on Sunday when an overnight bus smashed into another vehicle carrying diesel in eastern Pakistan, officials said.

The accident took place on a motorway at around 4.30am (2330 GMT on Saturday) near the town of Pindi Bhattian in eastern Punjab province. 

"Within five minutes, the whole bus was engulfed in flames. Eighteen people burned alive whereas 16 others were injured, of whom four are critical," senior police officer Fahad Ahmed told AFP on the phone. 

"It seems that the driver of the bus dozed off." 

The Suzuki open-back van was carrying drums of diesel and petrol.

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 January, at least 40 people died when a bus plunged off a bridge in southwestern Pakistan and burst into flames.