A burst oil pipeline has dumped thousands of litres of diesel fuel into a major tributary of China's Yellow River, threatening to contaminate the key watercourse.

State media reported the oil spill on the Chishui river in Shaanxi province on Sunday, although the ruptured pipeline - operated by state oil giant PetroChina - had been discovered on Wednesday.

Oil was detected as much as 33km downstream from the leakage site, the official Xinhua news agency quoted the local authorities as saying.

Emergency measures were being taken to halt the oil's spread, with 23 containment belts set up downstream from the spill and up to 700 people working to clean up the pollution, the government said.

Xinhua reported that up to 150,000 litres had leaked into the river from the pipeline.

In a statement on its website, PetroChina's parent company China National Petroleum Corporation (CNPC), said the leak was caused by a "third party" during construction work.

The pipeline is supposed to transport diesel from northwest China's Gansu province to central parts of the country.

Concern over reporting

Concern has been voiced about the amount of time between the contamination's discovery and its reporting by local media.
 
"There has been zero coverage of the accident on television or in newspapers in Chishui township or Hua county," the Hong-Kong based South China Morning Post quoted one local as saying.

Most of China's lakes and rivers are thought to be heavily polluted. Government data has found that more than 200 million Chinese 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 pullution then flowed down river into Russia, sparking a diplomatic crisis between the two nations.