GUANGZHOU - Authorities in South China's Guangdong province said Sunday they were working to clean oil slicks on a river which might threaten drinking water supply to a four-million-people city.

A water supply company in Jiangmen city reported Saturday afternoon that a wide range of oil was found floating near the source of the city's two waterworks on Xijiang River, said an official with the city government's information office.

The oil, possibly diesel, was believed to have spilled from a ship which sunk in the river late Friday, said the official.

Clean-up and pollution-control measures have been taken since Saturday afternoon and the government said, based on the results of repeated checks of water quality, tap water in Jiangmen is safe to drink.

The government pledged for further clean-up of oil spill and continued monitoring of water quality. Investigation of the accident is under way.

Xijiang River is a major tributary of the Pearl River, the third longest river in China and a pivotal waterway that runs across the economic heartland of Guangdong.