A bus careered off a road in mountainous southwest China and plunged into a valley on Saturday, killing all 51 people known to be aboard, state media reported, citing local officials.

The bus, which was travelling from the city of Bazhong in Sichuan province to Ningbo in eastern Zhejiang province, went off the road in a mountainous area, Xinhua news agency quoted a local official as saying.

The official said 51 people were on board when the bus left Bazhong, but it was not immediately clear if other passengers had been picked up along the way.

Witnesses said they heard an explosion and saw thick smoke rising from the site of the crash, the report said.

Sichuan provincial governor Jiang Jufeng was headed to the site to supervise the search and rescue operation, but so far, no survivors had been found, Xinhua said.

China's roads are among the most dangerous in the world, and bus drivers routinely cram passengers into their vehicles to get as much business as they can in one trip.

There were more fatalities on roads in the country last year than anywhere else in the world, according to an earlier Xinhua report, with more than 81,000 people killed, an average of around 223 a day.