Patches of blue-green algae began to appear last week in the Xinlicheng Reservoir, one of the city's major water sources for its 100,000 residents.

On Tuesday, the water supply from the reservoir was shut off indefinitely.

Changchun used 770,000 tons of water a day before the outbreak, and usage has plunged by 50,000 tons since the reservoir was taken out of service.

Experts said it will be difficult to remove the algae, but rainfall and cool weather may help disperse it.

The Changchun meteorological department has fired rockets into the air above the reservoir to create artificial rain in an effort to curb the outbreak.

The city government has set aside 10 million yuan (US$1.32 million) to clean up the water source, a government spokesman said yesterday.

Changchun Mayor Cui Jie said the money will be spent on extending pipes linking the city to another reservoir and transporting water with fire engines.

"We must do our best to ensure the drinking water supply for our citizens," said Cui, adding that he was considering closing car washes.

The algae outbreak in Chuangchun followed similar problems in other parts of China, including Taihu Lake and Chaohu Lake in east China and in Dianchi Lake in the southwest.

In May, the algae in Taihu Lake polluted the drinking water supply for more than two million residents in the lakeside city of Wuxi.