More than 50 mines and chemical plants in the city of Ma'anshan, in east China's Anhui Province, have been shut down by the provincial government for polluting the Cihu River, a small tributary of the Yangtze River.

The factories will not be allowed to operate again until they meet environmental protection standards, said Ding Haizhong, the city's Party chief.

The river is north of Ma'anshan and is an important water source of the city.

The city government investigated public complaints and found the pollution primarily came from the mines and chemical plants on the upper reach of the Cihu.

The pollution discharged by the factories has affected the safety of drinking and irrigation water, Ding said.

The pollution in China's longest river, the Yangtze, has worsened over the last 10 years as billions of tons of waste continue to be dumped into China's "mother river."

A report by the Yangtze River Water Resources Commission shows 30 billion tons of polluted water were dumped into the river last year, 50 percent more than in 1998.

About 27.5 percent of the river's water is too seriously polluted to be made potable, said the report.