Fifty dead pigs, including some decaying ones, were found Tuesday in a tributary of the Yangtze River in central China's Hubei Province, China Central Television reports.

Decaying pig carcasses float in the Wulong River, a branch of the Yangtze River, in Yichang, central China's Hubei Province, on Tuesday, March 12, 2013. [Photo/Image China]

Local authorities in the city of Yichang rushed to the scene along Wulong River to investigate the incident.

It was not clear where the pigs came from or whether the river had been contaminated.

In a related development, Shanghai officials said Tuesday that the number of pig carcasses found in the city's Huangpu River had risen to nearly 6,000.

Shanghai is located along the lower reaches of the Yangtze River bordering the East China Sea. The Huangpu River which runs through the city is a major source of local drinking water.

In a statement, the Shanghai municipal government said the water in Huangpu River was safe. It also said that no diseased pork had been detected in markets.

The government statement also said the number of pig carcasses salvaged from the river appeared to be decreasing.

Meanwhile, amid media speculation, agricultural officials in the nearby city of Jiaxing, Zhejiang Province, said they had not excluded the possibility that the dead pigs may have come from their city, but there had so far been no reports of any epidemic outbreak there.

Jiang Hao from Jiaxing's animal husbandry department said multiple factors were related to the pigs' deaths, including their raising conditions, techniques and climate.

Jiaxing is home to 100,000 pig farmers who raise a total of seven million pigs at present, according to data obtained by China News Service.

One local farmer who owns about 600 pigs said the animals are susceptible to illnesses during the winter, especially the young ones. But he added that villagers like him would probably not dump any animal carcasses into rivers.

"I would be severely punished for doing that," he said. "There are designated pools and garbage bins to dispose of dead pigs. I can also call local officials to help me deal with them. That will be more convenient than dumping them into rivers."