"Due to this incident, the Chishui river was seriously polluted and the Wei river was relatively seriously polluted," the Shaanxi provincial government said in a statement posted on its website on Tuesday.

But it said pollution on the Wei had been "effectively controlled".

The Yellow River is one of China's longest and a key watercourse that provides drinking water for millions of people including the population of eight cities downstream from the oil spill, the Xinhua news agency said.

Diesel spill

Xinhua said diesel was detected in water in the Sanmenxia reservoir on the Yellow River in neighbouring Henan province, prompting emergency measures to be taken to halt the oil's spread.

The measures included 23 containment belts set up downstream from the spill and up to 700 people working to clean up the pollution.

Officials said the pollution in the Wei river had been "effectively controlled" [AFP]
Authorities also shut down electricity production on the dam in an effort to keep the contamination from flowing downstream to the cities of Zhengzhou and Kaifeng.

On Monday the water quality on the tributaries had reached grade five, the worst level in China's pollution index.

Government standards stipulate that water on a level five quality is unfit for drinking but can be used for agricultural purposes.

More than 30 years of unbridled economic growth have left most of China's lakes and rivers heavily polluted, while the nation's urban dwellers also face some of the world's worst air pollution.

Government data shows that more than 200 million Chinese currently do not have access to safe drinking water.

In November 2005, a major oil spill on the Songhua river in northeast China resulted in a cut off of water supplies to up to four million people.

The pollution then flowed down river into Russia, sparking a diplomatic crisis between the two nations.