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.