HANOI - At least 34 people were killed when a crowded overnight bus plunged off a bridge and partially submerged into the Serepok river in Vietnam a Southeast Asian country known for one of the world's highest traffic fatality rates.

The driver of the 50-seat coach apparently lost control over the wheel and ripped the bus through the bridge's guardrails Thursday night diving about 18 metres, local official Tran Bao Que said.

The coach was travelling from Buon Ma Thuot in central highlands province of Dak Lak to the southern city of Ho Chi Minh.

Y. Bliu Arul, deputy director of the General Hospital in Dak Lak, said the two bus drivers were among the 32 people who died at the scene. Two more died in the hospital.

Of the 21 people injured, 16 were said to be in a serious condition.

Vietnam has one of the world's worst rates of traffic accidents, with an average of 33 people dying on its roads every day, according to official records.

Nguyen Van Khanh one of the survivors said everyone in the bus was sleeping when the accident happened.

"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," he told news website Dan Tri.

Another survivor Trinh Van Mui, 34, said he was travelling with his three-year-old daughter. He recalled hearing a big boom before he was knocked unconscious. "I later found I was in the hospital with pain over all my body. We were very lucky to survive," he told The Associated Press.

Officials said rescuers took about four hours to pull the bodies from the bus and had to use axes to try to free trapped passengers.

The cause of the bus crash was not immediately known but authorities said they were investigating.