At least 20 people in China were killed Friday when two trucks collided and sent petrol into a timber mill below the road, causing a massive blaze, authorities said.

The accident happened before dawn near the southern city of Guangzhou, according to a statement posted on the microblog of the city government's official news portal.

It said 20 people had been confirmed killed -- most of them believed to be workers in the timber mill -- and 14 injured, one of them seriously.

The two trucks were involved in a rear-end collision on a bridge road, the statement said, but it was unclear whether the tanker, which was carrying 40 tonnes of petrol, first ploughed into the other lorry, or vice versa.

The statement said petrol gushed out of the truck and into the timber mill underneath the bridge, causing a fire. Firefighters evacuated the site and the blaze was brought under control in a matter of hours.

Photos of the accident posted online showed charred bodies lying on the floor next to burnt vehicles. One picture taken from afar during the blaze showed smoke rising high above the skyline of a nearby city.

Vehicles carrying explosive materials must first get permission from the police before travelling on the roads in China. The microblog statement said the exact cause of the accident was being investigated.

China's roads are highly dangerous, with traffic laws and safety widely flouted, and truck drivers typically overworked.

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