A diesel spill from a ruptured pipeline in northwestern China has seriously contaminated two rivers that finally flow into the Yellow River, the country's second longest waterway, a local official said Monday.

The Chishui and Weihe rivers were seriously contaminated after some 150 cubic meters of diesel leaked early Wednesday from a ruptured pipeline belonging to the China National Petroleum Corporation (CNPC), the country's top oil producer, Li Xiaolian, vice director with the Shaanxi Provincial Environment Protection Administration, said at a press conference.

However, contamination to the Yellow River remained under control and its water quality was still within the state standard, he said.

The leak occurred in Huaxian County in Shaanxi, about three kilometers away from the confluence of Chishui and Weihe rivers and 70 kilometers upstream from the Yellow River.

Statistics from a monitoring station in Tongguan County, where Weihe River joins the Yellow River, indicated Monday morning that each liter of the water contains about 0.79 milligram of oil-like substance, still below the national limit of 1 mg per liter.

The figure showed continuous declining of oil substance in the water compared to the 1.78 mg concentration reported Friday noon at Tongguan.

Nine such stations have been set up along Chishui and Weihe rivers to continuously test water quality.

The CNPC had dug a 100-meter-long chanel at the source of the leak to divert uncontaminated water.

Test results showed that the contamination has affected the section of the Yellow River in Sanmenxia city, central China's Henan Province.

The provincial government said late Monday night they have built several barriers at the Sanmenxia Yellow River bridge, and near the dam of the Sanmenxia Reservoir to block the contamination from spreading.

Thanks to preventive measures, sources of drinking water for urban residents and water downstream the Sanmenxia Reservior were not contaminated, according to the provincial government. It said, the quality of water flowing into Henan from upstream is returning to normal gradually.

The provincial government has urged all cities along the Yellow River to take preventive measures to ensure water quality and supply for local residents.

Henan has eight cities along the middle and lower reaches of the Yellow River, of which Zhengzhou, the provincial capital, and Kaifeng City take the Yellow River as major drinking water source.

About 95 percent of the tap water in Zhengzhou, a city of 2.4 million, come from the Yellow River, said Zhang Jianyu, deputy head of the work safety department of the Zhengzhou Tap Water Company.

If the Yellow River water is seriously contaminated, reservoirs with a total water reserve of 24.5 million cubic meters in Zhengzhou could provide the city with safe drinking water for 50 consecutive days.

And the two reservoirs in Kaifeng City could also ensure the water supply for the 800,000 citizens for more than half a month, said a spokesman with the city's tap water company.