Thirteen people died and 15 others were injured when a passenger bus plunged into a ravine beside an expressway in Southwest China's Yunnan province Monday afternoon, local authorities said.

The accident happened at 4:25 pm on a section of the Dali-Baoshan Expressway in Laoying township in the city of Baoshan, when 29 people, including the driver, were on board, according to the city's publicity office.

The passengers, mostly residents from a village in Nanhua County, Chuxiong Yi autonomous prefecture, were on a tourism journey, it said.

The injured have been rushed to local hospitals, with two in critical condition.

An initial investigation showed that the driver was responsible for the accident, as he was operating the vehicle improperly in heavy rain, the Baoshan Municipal Public Security Bureau said.

Further investigation into the cause of the accident is underway.