Twenty people were killed in an explosion inside an under-construction expressway tunnel in central China's Hunan province on Saturday, state media said.

The blast happened in Zhuzhou city when a vehicle carrying explosives was unloading in the tunnel, the Xinhua news agency said, citing local authorities but providing few more details.

Four workers had been pulled out of the tunnel, including one in a critical condition, the report said.

China's roads are notoriously dangerous, with traffic laws and safety widely flouted. Bus operators often seek to cram as many people as they can into their vehicles to maximise profits and drivers hurtle down highways.

Last year alone, more than 62,000 people died in traffic accidents, state media said, citing police figures.