The local Guangdong provincial government issued the order to Guangzhou and neighbouring Foshan city, the Xinhua news agency said.

The incident follows a chemical spill in a river in northeast China last month that left millions without water for four days, highlighting the seriousness of water pollution in China and raising questions about Beijing's ability to handle its rapid pace of development.

The latest toxic slick was caused by an excessive discharge of cadmium from a state-owned smelting works in the Beijiang river, a major source of drinking water for cities in the northern part of Guangdong, Xinhua said.

The Beijiang runs into the Pearl river which flows through Guangzhou.

Waste discharges increased the volume of cadmium in the Beijiang at Shaoguan city to nearly 10 times above safety levels, "seriously endangering" the safety of water downstream, Xinhua cited the local government saying.

It did not say when the discharge occurred, but provincial environmental officials were sent to the area Sunday, Xinhua said.

Local governments have set up monitoring posts along the Beijiang river to check water quality, it said.

Officials in Yingde, a city with a population of one million some 90 kilometers (55 miles) downstream from Shaoguan, told AFP Wednesday they had lowered a dam gate to block the slick from flowing to the part of the river supplying water to Yingde's urban areas.

They were now working around the clock to build a pipeline to divert clean water from a local reservoir, to provide potable water to the 100,000 residents there.

"We've worked the whole night last night and expect to complete it in 48 hours," said an official from Yingde's Water Resources Bureau.

Cadmium is a chemical used in protective plating. Serious exposure can cause diarrhoea, stomach pains, severe vomiting, bone fracture, reproductive failure and damage to the central nervous system and the immune system.

Guangdong provincial government has decided to release water from a reservoir in the upper reaches of the river to dilute the pollution so that the water will be safe enough to drink, Xinhua quoted experts saying.

Water carriers, including 15 fire engines, were also ferrying in drinking water, it said.

A Yingde city official told AFP the city was confident it would not need to cut off water.

"There are three rivers flowing to the city. Even if Beijiang's water is unsafe, we can use water from the other rivers," said the official.

Polluted water blocked by the dam gate will be treated and then diverted elsewhere including to irrigate farmland.

The spill has polluted the river water in Shakou, a town to the north of Yingde, but Shakou officials told AFP the town of 40,000 does not rely on river water.

"We are advising the people living on both banks of Beijiang to not allow their farm animals to drink the Beijiang water," a Shakou official said.

Last month's spill in northeast China's Heilongjiang province was caused by an explosion at a benzene plant. That spill is headed to the Russian city of Khabarovsk along the Amur river and predicted to hit the city of 600,000 early Thursday.

Many factories in China are located on river banks as they need water in their production process and are notorious for discharging untreated industrial wastes into the rivers, causing most of China's river water to be undrinkable.

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