The local government of the Tibet autonomous region has put a cap on the number of passengers allowed on tour buses on the region's mountainous roads after two recent deadly accidents involving buses.

"It's for safety," said a man who answered the phone at the Tibet Regional Tourism Bureau on Wednesday but refused to give his name.

He confirmed that the new rule puts a 20-person limit - including a driver, a guide and a police officer - on any tour vehicle in Tibet.

Tourism has been booming in the Himalayan region, which also is home to some treacherous roads that wind and dip amid steep mountains.

A tour bus carrying about 50 people plunged into a ravine on Aug 9 after colliding with a pickup and a sport utility vehicle in southern Tibet. Forty-four people died.

On Monday, a 45-person tour bus fell into a river in southeastern Tibet, killing three and leaving 13 others missing.

A man at the Lhasa Youth Travel Service, who gave only his surname of Wang, said he received a notice announcing the limit on Tuesday. A woman at the Lhasa International Travel Service also confirmed the same directive, saying the recent traffic accidents prompted the size restriction on traveling groups.

Li Simin, an expert on tourism, said the measures, including the addition of a police officer on board, will improve safety.