A green algae outbreak has been reported in China's fifth-largest freshwater lake, threatening the drinking water source for 300,000 residents in an eastern city.

Three to four square km of green algae were found on the east part of Chaohu Lake, the water source for Chaohu city, Su Huimin, chief of Chaohu municipal environment protection bureau, said Thursday.

"Hot weather and flood discharge from the more polluted western Chaohu

Lake have led to the algae outbreak," said Su.

Su told Xinhua that the drinking water supply for local residents has not been affected so far, but the green algae was close to the city's drinking water source.

"We will now test the water quality every two hours, instead of every four hours as before," said Tang Xiaoxian, director of the local environment monitoring center.

Tang added that local authorities have beefed up monitoring of the green algae and also sent boats to clear it.

Covering an area of 13,000 square km, Chaohu suffers from severe environmental pollution, with lake embankments destroyed and wetland damaged during the urbanization and industrialization drives in the province.

In 2007, an algae outbreak in China's third-largest freshwater lake, Taihu, cut tap water supplies for more than 1 million people in Wuxi, Jiangsu Province for about 10 days.