Thirty-two people, including 11 children, were killed and about 20 others injured when the bus they were travelling in collided with an oil tanker in Pakistan's southern Sindh province, officials said on Sunday.
The collision late last night near Hyderabad city triggered a massive fire as the tanker was loaded with around 40,000 litres of fuel.
Reports said the bus rammed into the back of the oil tanker parked by the highway after the driver apparently fell asleep.
About 50 people, including women and children, were in the bus, regional police officer Muhammad Farooq said at the accident site.
"The driver lost control of the bus and hit an oil tanker from the back side. 32 people have been killed and several others injured," Farooq told reporters.
"There are women and children among the dead. Some dead bodies are beyond recognition," he said.
Rescue service officials said many of the bodies were charred beyond recognition.
Farooq said both vehicles caught fire and people died of burn injuries. He said fire fighters were late in arriving at the accident site and people sitting at the front of the bus could not get out because of the flames.
"As the bus caught fire, people trapped inside it were screaming. They could not get out. Then the fuel tank of the bus exploded," an eyewitness told the media.
The people in the back of the bus managed to get out by smashing windows. They were taken to a nearby hospital where about 20 injured people were treated by doctors.
The injured said the bus driver was speeding even though he was drowsy.
They said some passengers asked the driver to be alert but he ignored their advice. The driver lost control of the bus and hit the tanker, they said.
The bus was heading to Sukkur in interior Sindh from Karachi, the provincial capital.
Footage on television showed the bus was gutted by the blaze.
Violation of traffic rules and inadequate safety standards are the main reasons for road accidents in Pakistan.
About 87 per cent accidents are caused by the negligence of drivers who indulged in speeding, overloading and wrong overtaking.