Beijing, China: A bus plunged 100 meters into a river in southwest China on Saturday killing 21 people, state media reported.

An additional three people were also injured when the bus carrying 24 passengers went off a mountain road onto a dry river bed, the official Xinhua News Agency said.

Fatal road accidents are a serious problem in China, where the highway network and number of new drivers are expanding rapidly while traffic laws and safety are widely flouted.

Investigators believe the bus was illegally using a road intended for smaller vehicles in Bijie city, Guizhou province, the report said.

Road accidents killed more than 87,200 people from about 426,000 accidents in the first 10 months of 2014, according to figures from the Ministry of Public Security, a 1.9 percent increase from the same period in 2013.

But a study published in 2011 by a group of Chinese and US researchers concluded that the number of deadly accidents was twice as high as police said.

In July last year, 43 people died when a van carrying inflammable liquid hit a bus on a motorway in central China.

In August, a tour bus plunged into a valley in Tibet after hitting two vehicles, leaving 44 people dead and 11 injured.

Road accidents is the third largest cause of death in China, killing more people than cancer, according to a study in the British medical journal The Lancet.