Environment officials in a south China city have assured residents that their water is again safe to drink after supplies to about 100,000 people were cut during an oil pollution scare on Saturday.

Domestic water supplies to about half the population in the urban area of Foshan city in west Guangdong Province were cut for more than six hours from 8:45am on Saturday after a two-kilometer long ivory-white slick was spotted in the Xijiang River, forcing restaurants and businesses to close and sparking a surge in bottled water sales.

Two pumps of the Gaoming Waterworks in Gaoming District were closed down while technicians cleaned their filters at a pumping station on the river, and officials began an emergency operation to soak up the oil and clean the river.

The city's marine affairs department also sent oil skimming boats to help clearing the pollutants.

Three other waterworks in charge of water supply to the urban areas of Foshan city were required to operate at full capacity to ensure the domestic water supply, while the city government has informed the citizens with cell phone text messages on the oil pollution.

The silt was cleared by 1:30pm and water supply resumed at around 3pm on Saturday.

"Tests show the water was safe to drink, but we will keep on monitoring the water quality of the river," said an official with the city's publicity department who just gave his surname as Li.

Environment officials have begun an investigation into the source of the pollution and the type of oil, which they said emitted a "strange unpleasant odor", but no pollutants were found in the upper reaches of the river.

The 2,075-meter-long Xijiang River, a major tributary of the Pearl River, runs through the western part of Guangdong. It is a major water resource for the cities of Foshan, Sanshui, Shunde, Zhongshan and Zhuhai.