At least 30 people have been killed when a large bus burst into flames after crashing into the side of a road in central China, state media says.

A total of 56 people were on board the bus which crashed in Hunan province, the official Xinhua news agency reported, suggesting casualties could rise.

Twenty-one injured passengers have been sent to hospitals, Xinhua said, adding that the driver of the bus had been detained.

Video posted online by state broadcaster CCTV showed thick black smoke billowing from the vehicle, as passengers appeared to stand nearby.

Local officials believe that an oil leak caused by the bus crashing into a roadside guard rail may have caused the fire, Xinhua said.

Traffic accidents are common in China, with more than 250,000 people dying each year on the country's roads, according to the World Health Organisation.

Long-distance buses are popular as a cheap way of travelling between cities and competition between bus firms can be fierce.