The algae outbreak in Changchun comes after similar problems in lakes in other parts of China.

But while appearances of blue-green algae in Lake Tai in eastern China and Dianchi Lake in southwestern China "were mainly attributed to pollutants from chemical factories," the problems in Changchun were the result of farm fertilizer, Xinhua News Agency said.

Blue-green algae, a plant-like organism, blooms when nutrients, sometimes caused by excessive pollution, build up in water. Some algae can produce dangerous toxins and if ingested can cause vomiting, respiratory failure and, on rare occasions, death.

Xinhua said the algae began to appear in Xinlicheng reservoir, one of Changchun's major water sources, last week.

The city government is using active carbon and chlorine to clear the water, and has dispatched workers to clean out the algae, it said.

Changchun has a population of more than 7 million, with 2.7 million living in the downtown area.

The outbreak of algae in Lake Tai forced 5 million residents of the eastern lakeside city of Wuxi to drink and bathe with bottled water.