A speeding, overcrowded passenger bus plunged from a highway into a rocky ravine in south-western Pakistan early on Friday, killing 40 people and injuring eight others in one of the country's deadliest road accidents in recent years, officials said.

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

He said the bus was carrying not only its own passengers but also travellers from another bus that had broken down, leaving the vehicle overcrowded. Rescue workers were still working to identify those killed in the crash, Rind added.

President Asif Ali Zardari expressed sorrow over the tragedy, offering condolences to the families of those killed and wishing the injured a speedy recovery.

He directed the relevant authorities to ensure the injured received the best possible medical care.

In a statement, Balochistan Chief Minister Sarfraz Bugti also expressed grief over the loss of life and instructed officials to provide the injured with the best possible treatment.

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

In May, a minibus crashed into a bus parked on a motorway in north-western Pakistan, killing 17 people and injuring five others.