More than 40 people have died when an express bus burst into flames on an elevated road in south-eastern China, reports say.

The blaze occurred during the evening rush hour in the port city of Xiamen, the official Xinhua News Agency said. At least 42 people died and 33 were injured.

A local fire official said bodies were piled up inside the bus. He said the cause of the fire was being investigated.

Operations of the entire express bus system, known as Bus Rapid Transit, were suspended after the accident. A survivor said she smelled petrol and then saw a fire spreading rapidly.

Photos on Chinese social media showed huge clouds of black smoke rising from the burning bus. Victims were seen bleeding with torn clothes and burned skin.

Witnesses reportedly heard sounds of explosions after the fire had been burning for about 10 minutes.

Xiamen's BRT system began operations in 2008. It covers more than 80 kilometres and carries more than 250,000 people each day. The buses are often crowded during rush hour.