Shanghai said it pulled hundreds more dead pigs out of its main river Wednesday, bringing the total to just over 6,600, as residents worried over drinking water criticised the handling of the incident.

Shanghai recovered 685 more dead pigs from the Huangpu river, which cuts through China's commercial hub and supplies 22 percent of its water, but water quality was within national standards, a government statement said.

The pig carcasses, believed to have been dumped by farmers upstream after dying of disease, have been hauled out of the river since Saturday but the numbers have begun to slow on a daily basis.

"Treated water meets national drinking water hygiene standards," the Shanghai government 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.

"So many dead pigs and the water quality has not been affected. Who are you fooling? Do you think people are idiots?" said one online posting under the name Youshan Wanshui6_6.

In another posting on a Twitter-like microblog, Shoppinggirl Caijiajia said: "The dead pig incident has completely ruined Shanghai's image."

"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, and on Wednesday an official admitted some of the dead pigs could be from the area.

"We do not rule out the possibility of dead pigs in Jiaxing's waterways floating into Shanghai," a Jiaxing agricultural official, who was not named, told an online chat on the website of the People's Daily newspaper.

"But as far as we grasp the current situation, there are dead pigs from areas outside Jiaxing also flowing in," he said.

The official added that a handful of tags found on the dead pigs in Shanghai had been traced to a local producer and police were bringing a case against the individual, who was not identified.

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 carcasses.

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.