Diesel oil was threatening to spill into China's Yellow River on Sunday, after a leak contaminated a tributary in the country's north-west, local media reported, dpa reported.
The leak, which was discovered in a pipeline on Wednesday morning, had already contaminated the Weihe River, located 73 kilometres upstream from the Yellow River, the official Xinhua news agency reported.
The pipeline owned by state oil giant PetroChina was shut down and more than 700 people were at the scene trying to control the spill and clean up the pollution, the report said.
Oil was detected 33 kilometres downstream from the leakage site, Xinhua quoted the local authorities as saying.
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.
There were concerns 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.
In 2005, toxic chemicals spilled from a CNPC petrochemical plant into the Songhua River in north-eastern Jilin Province caused massive water contamination and affected millions of people in Harbin and Russian cities downstream.
Then-environment minister Xie Zhenhua resigned amid criticism of negligent handling of the crisis, including failure of ecological authorities to sound the alarm immediately after the spill.