The Yellow River, the second-longest in China, has seen its water quality deteriorate rapidly in the last few years, as discharge from factories increases and water levels drop because of diversion for booming cities.

The river supplies a region chronically short of water but rich in industry.

The Yellow River Conservancy Committee said 33.8 per cent of the river's water sampled registered below level 5, meaning it's unfit for drinking, aquaculture, industrial use and even agriculture, according to criteria used by the United Nations Environmental Programme.

A 2007 survey covered more than 8,384 miles (13,493 kilometres) of the river, which flows from western Qinghai province across China into the Bohai sea, and its tributaries, a notice posted on the committee's website on Saturday said.

Only 16.1 per cent of the river samples reached level 1 or 2 - water considered safe for household use.

Industry and manufacturing made up 70 per cent of the discharge into the river, the notice said, with 23 per cent coming from households and 6.4 per cent from other sources. The notice did not identify specific pollutants.

Some of the world's most polluted cities are in China, where many rivers and lakes are toxic after decades of breakneck industrial and economic growth.

In February pollution turned part of a major river system in central China red and foamy, forcing authorities to cut water supplies to as many as 200,000 people.

In one of China's worst cases of river pollution, potentially cancer-causing chemicals, including benzene, spilled into the Songhua River in November 2005. The north-eastern city of Harbin was forced to sever water supplies to 3.8 million people for five days.

Pollution in China's waterways remains 'grave', according to a June report by the Ministry of Environmental Protection on the state of the environment in 2007. More than 20 per cent of water tested in nearly 200 rivers was not safe to use, it said.

The ministry has tried to shut down polluting factories along China's main waterways, but its power is limited because local environmental protection bureaus are under the control of local governments. -- AP