QUETTA, Pakistan — A speeding, overcrowded passenger bus plunged from a highway into a rocky ravine in southwestern Pakistan early on Friday, killing at least 40 people and injuring eight others in one of the country’s deadliest road accidents in recent years, officials said.

The bus went out of control and fell into the ravine in Dana Sar, a remote area near the border of Balochistan and Khyber Pakhtunkhwa provinces, said Shahid Rind, a spokesman for the Balochistan government.

He added that the bus was carrying not only its own passengers, but also those of another bus that had broken down, making the vehicle overcrowded.

Rescuers are working to identify those killed, the spokesman said.

An injured survivor told local media from his hospital bed that some passengers protested after the driver stopped to pick up people from the bus that had broken down and was also headed to the city of Peshawar.

An argument followed, the survivor said, during which one passenger allegedly grabbed the driver by the neck. Moments later, the driver lost control of the bus, which plunged into the ravine.

The passenger’s account could not be independently verified. Police said they were still investigating.

A regional government administrator, Hazrat Wali Kakar, said rescuers transported the victims to nearby hospitals.

Rescue officials said the bus had 48 passengers when it crashed.

The eight injured passengers initially received medical treatment at the scene before they were taken to the district headquarters hospital in Zhob, while the bodies of those killed were brought to a district hospital, officials said.

Shah Fahad, director general of rescue emergency services in Khyber Pakhtunkhwa, said his agency was working with its counterpart in Balochistan in rescue and recovery efforts.

If any of the dead are confirmed to be Khyber Pakhtunkhwa residents, authorities will transport their bodies by ambulance to their hometowns and assist their families, he added.

Pakistan’s President Asif Ali Zardari expressed sorrow over the crash, offered condolences to the families of those killed, and wished the injured a speedy recovery.

He directed the relevant authorities to ensure the injured receive the best medical care possible — a move mirrored by Balochistan Chief Minister Sarfraz Bugti, who also voiced grief over the tragedy in a statement.

Road accidents are common in Pakistan because of poor road conditions, inadequate enforcement of traffic laws, and unsafe driving practices, particularly in mountainous areas.

In May, a minibus rammed into a bus parked along a motorway in northwestern Pakistan, killing 17 people and injuring five others.