Shanghai's tap water has been given the all-clear despite the supply in neighboring Jingjiang, Jiangsu Province, being suspended for almost seven hours yesterday, officials said.

Hundreds of thousands of people were affected in the east China city after authorities cut the supply after samples taken from the Yangtze River produced abnormal readings, Xinhua news agency said.

The Jingjiang government did not elaborate, but said on its official microblog post that it had "launched an emergency response plan."

The Jingjiang tap water company cut the supply to 680,000 people at 10am yesterday, immediately after its workers reported a strange smell coming from the Yangtze.

An online rumor claimed that a ship carrying pesticides had capsized in the river, but the Jiangsu maritime authority ruled out such an event, China News Service reported.

Despite residents clearing their local stores of bottled water, the mains supply was resumed at 4:40pm, the Jingjiang publicity department said.

The alternative source is an ecological park, which has sufficient resources to meet people's needs for about a week, it said.

Shanghai residents were told that samples taken yesterday afternoon from Qingcaosha Reservoir at the mouth of the Yangtze, which supplies most of the city, and other local sources had tested normal.

However, officials said it could take up to two days for the possibly contaminated river water to reach Shanghai.

As a precaution, the city government said it has strengthened its monitoring of water quality and boosted reservoir storage.

According to officials at the Ministry of Water Resources, no signs of contamination were found in either Taihu Lake or the Huangpu River.

Shanghai gets some of its tap water from the upper reaches of the Huangpu River.

Officials in other cities in the higher reaches of the Yangtze said they are closely monitoring the situation, though no contamination was reported in Jiangyin, which faces Jingjiang across the Yangtze, the local government said.

Yesterday's incident came almost two weeks after a water contamination case in Wuhan, capital of central China's Hubei Province.

On April 23, two supply plants in the city suspended operations after tests showed the Wuhan section of the Hanjiang River, a tributary of the Yangtze, contained excessive levels of ammonia and nitrogen.

A third plant was shut down the following morning, but the supply was reinstated hours later.

An investigation found that the pollution had been caused by dirty water being discharged through two gates on the upper reaches of the Hanjiang.

Also last month, in Lanzhou, Gansu Province, levels of the cancer-inducing chemical benzene in the tap water were found to be 20 times the national safety level.