Police are investigating the cause of a deadly accident in Vietnam, in which 34 people were killed and 21 injured.

A passenger bus, which was travelling from the city of Buon Ma Thout to southern Ho Chi Minh City, overturned, plunged off a bridge and crashed into the Serepok river in one of the country's deadliest road accidents.

Local official Tran Bao Que told AP that the 50-seat coach fell about 18 metres and landed on its roof after the driver apparently lost control.

"When the accident happened, everyone in the bus was sleeping," survivor Nguyen Van Khanh told the news website Dan Tri. "I vaguely heard a noise like gunfire and then people were screaming when the bus was overturned... I managed to escape through a window that was smashed opened by others."

Rescuers took four hours to pull the bodies from the bus, using axes to free trapped passengers. Graphic pictures show a body hanging out the side of the vehicle after it was ripped open by the crash."The front head of the bus was submerged and deformed, making it hard for the rescue work," Daklak's deputy chairman Dinh Van Khiet said.

Two drivers were among the 32 people who died at the scene. Two others died later after being taken to the local hospital.

Traffic accidents are common in Vietnam and kill an average of 35 people every day, due to poor road conditions, old vehicles and carelessness regarding road safety and traffic rules.

Rescuers search for victims inside a bus that crashed in Vietnam's central highland province of Daklak (Reuters)