Diesel oil leaking from a pipeline owned by state oil giant PetroChina has contaminated a major river in Shaanxi province, threatening water supplies for hundreds of thousands of people, state media reported. The leak, which local cadres described as huge, was discovered early on Wednesday in Chishui township, Hua county, site of a big tourist attraction, Hua Mountain. The Wei River, a major tributary of the Yellow River running through the province, has been contaminated. Within hours, Vice-Premier Li Keqiang ordered urgent steps to deal with the leak. A Shaanxi Television reporter, who declined to be named, said Li urged the Ministry of Environmental Protection, PetroChina's parent company, China National Petroleum Corporation (CNPC), and local governments to work together to stop the leak as soon as possible and do their best to prevent pollution of the Yellow River. Xinhua said the CNPC had shut off the pipeline, part of a 1,188-kilometre network carrying oil from Lanzhou in the northwestern province of Gansu to Zhengzhou in the energy-hungry central province of Henan . More than 700 people were still at the scene yesterday trying to control the spill and clean up the pollution, it said. Xinhua did not provide further details about the scale of the accident or even its exact location. Its report merely said that 'a large amount of oil has spilled into the Wei River and its tributary the Chishui' and that authorities had worked hard to secure the water supplies to people living nearby. The report did not explain why it took four days for news of the leak to emerge. CNPC said on its website that the leak was caused by an accident during construction work by a 'third party', probably a reference to subcontractors. No one from CNPC was available for comment yesterday. The pipeline, completed last year, will eventually be connected with a line running from the northeast to form the mainland's biggest oil pipeline network. Hua county has a population of 340,000 and the city of Weinan , which administers the county, a population of 5.4 million. An environmental official in Hua county confirmed Xinhua's report but said the spill was under control and it did not create severe pollution. 'We have worked day and night for the past few days to tackle the leak and all I can say at the moment is that it has not affected drinking water supplies in our county,' the official said. Local residents complained of a media blackout of the accident. 'There has been zero coverage of the accident on television or in newspapers in Chishui township or Hua county,' one said. 'I happened to hear about it when I met local cadres on Wednesday and they said it was a huge accident.' In 2005, local authorities covered up a spill of toxic chemicals from a CNPC petrochemical plant in the northeastern province of Jilin into Songhua River to 'maintain stability'. But it soon escalated into one of the mainland's worst pollution incidents in decades, causing massive water contamination and affecting millions of people in Harbin and Russian cities downstream. Dozens of central and local government officials were punished, and Xie Zhenhua , director of the State Environmental Protection Administration, was sacked.