More than 1,000 residents in Chifeng city of the Inner Mongolia autonomous region have fallen ill from drinking water tainted by a polluted lake, local health authorities said Tuesday.

More than 1,100 residents of Chifeng's Xincheng district were treated for drinking the contaminated water while 15 others were hospitalized for observation, the Xinhua News Agency reported.

The heavy rain raised the water in a nearby lake, which in turn flowed into and contaminated aquifers where drinking water is stored, Xinhua reported.

Everyone who got sick from drinking polluted tap water will receive free treatment in hospital, Mayor Wang Zhonghe told local media on Monday.

The local water supply company cut off the tainted water sources in Xincheng district last Friday and suggested residents boil water before using it.

Two days after the shutdown, the Chifeng health department received reports from Xincheng district community service center that many patients with gastrointestinal illnesses went to see doctors.

Most patients came from eight downtown communities in Chifeng and suffered from fever, diarrhea or vomiting after drinking tap water at home last weekend.

Chifeng hospital set up a special department for diarrhea patients in Xincheng district on Monday, a doctor surnamed Feng in Chifeng hospital said yesterday.