The number of dead pigs found in Shanghai's main river has doubled in two days to nearly 6,000, the government said, as residents worried over the water supply questioned the handling of the incident.

Shanghai had pulled 5,916 dead pigs out of the Huangpu river, which cuts through China's commercial hub and supplies 22 percent of its water, since Saturday, the local government said in a statement late Tuesday.

The number of pigs taken out of the river -- believed to have been dumped by farmers upstream after dying of disease -- had started to fall on a daily basis, it added, and water quality was within national standards.

"The water quality of the upper reaches of the Huangpu river is generally stable, basically similar to the same period last year," the statement said.

But residents questioned the government assurances and called for more transparency over the incident, which has received international attention and challenged Shanghai's image as one of China's most advanced cities.

"Nearly 6,000 dead pigs and the water quality is still stable. That really is a miracle," said a Shanghai resident under the name pinpo_x9x in an online posting.

"The dead pig incident has completely ruined Shanghai's image," said Shoppinggirl Caijiajia on her Twitter-like microblog.

"Without punishment, without accountability, how can it be guaranteed that this kind of thing won't happen again? Please give the Shanghai citizens drinking the dead pig water a clear explanation," she added.

Shanghai has pointed the finger at Jiaxing in the neighbouring province of Zhejiang, a major centre for hog-raising, but officials from the area sought to deny it was the source.

Jiaxing officials said investigations were continuing, the Shanghai Daily newspaper reported on Wednesday.

"We don't exclude the possibility that the dead pigs found in Shanghai were from Jiaxing. But we are not absolutely sure," Jiaxing spokesman Wang Dengfeng told a news conference.

"It is unclear where the dead pigs were raised, thus the dead pigs might be from elsewhere," he said.

Shanghai had handed the Jiaxing government ear tags from some of the dead pigs to verify their origin, media reports said.

Shanghai's agricultural commission said on Monday that some of the animals had tested positive for porcine circovirus, which it described as a common swine disease that does not affect humans.

Local television showed a backhoe digging a mass grave and then dumping dead pigs from a cargo net into the hole, as a worker in a white bio-hazard suit poured disinfectant on the bodies.

The city has tightened supervision over its markets to avoid tainted meat from the dead pigs being sold to consumers, the Shanghai Daily said.

Meat producers in China sometimes sell animals that have died from disease, instead of disposing of them, amid lax food safety laws.