BEIJING - China's longest river is "cancerous" with pollution and rapidly dying, threatening drinking water supplies in 186 cities along its banks, state media said on Tuesday.

Chinese environmental experts fear worsening pollution could kill the Yangtze river within five years, Xinhua news agency said, calling for an urgent clean-up.

"Many officials think the pollution is nothing for the Yangtze," Xinhua quoted Yuan Aiguo, a professor with the China University of Geosciences, as saying.

"But the pollution is actually very serious," it added, warning that experts considered it "cancerous.""

Industrial waste and sewage, agricultural pollution and shipping discharges were to blame for the river's declining health, experts said.

The river, the third longest in the world after the Nile and the Amazon, runs from remote far west Qinghai and Tibet through 186 cities including Chongqing, Wuhan and Nanjing and empties into the sea at Shanghai.

It absorbed more than 40 percent of the country's waste water, 80 percent of it untreated, said Lu Jianjian, from East China Normal University.

"As the river is the only source of drinking water in Shanghai, it has been a great challenge for Shanghai to get clean water," Xinhua quoted him as saying.

China is facing a severe water crisis -- 300 million people do not have access to drinkable water -- and the government has been spending heavily to clean major waterways like the Yellow, Huaihe and Yangtze rivers.

But those clean-up campaigns have made limited progress because of spotty regional enforcement. Toxic spills are common, the worst recently being in the Songhua river in the northeast which led to the taps of Harbin being turned off for days.

Despite immediate concerns for the cities along its banks, the Yangtze, along with the Yellow river, is earmarked for China's ambitious South-North water diversion scheme -- a plan to pump water from southern waterways to the parched north.

But environmentalists fear that unless local governments and industries start getting serious about cutting pollution, most of the water shipped north will not be fit to drink.

Most of the Yellow River, the second-longest in China and the cradle of early Chinese civilisation, is so polluted it is not safe for drinking or swimming, Xinhua news agency said in May last year.