The accusations came after at least 21 dead birds, including ducks and chickens, washed up on beaches in Hong Kong. At least three of them have tested positive for the H5N1 avian flu virus.
The Hong Kong government has said that there are no bird farms near where the corpses washed ashore and that it is liaising with the Chinese authorities to determine if they came from the Pearl River delta region.
There have been eight cases of bird flu infecting humans in January alone, five of which have been fatal. The cases have been distributed widely throughout China and there is a growing fear that contaminated poultry has entered the food chain.
There have been unconfirmed rumours that infected chickens in Jiangsu province were bleached with hydrogen peroxide and sold. There was a mass cull of 300,000 birds in December after an outbreak.
Lo Wing-Lok, a bird-flu expert in Hong Kong, warned that "something very terrible" could be happening in China. "There's no doubt there has been an outbreak, but the government has not admitted it," said Mr Lo.
The United Nations Food and Agriculture Organisation also said that there "must" have been outbreak among chicken, duck or geese flocks.
"There must have been some virus circulation or possibly some outbreaks lately," said Dr Vincent Martin, an FAO senior technical adviser on bird flu. "There are more cases than last year, including in places where the disease was not reported before," he said, but added that the FAO had not been told about any further problems since the Jiangsu incident.
Since bird flu rarely passes from human to human, the victims in January must have been infected by eating contaminated meat, he noted.
"There must be some suspicions of the disease reported to the ministry of agriculture, and some of those suspicions might have come up positive," he said. "We are waiting to receive some reports describing the overall epidemiological situation and the results of investigations."
The World Health Organisation declined to comment on whether or not there had been a cover-up by the Chinese government. China was widely criticised for suppressing and distorting the true extent of the SARS outbreak in 2003, when officials in Guangdong covered up the epidemic for almost six months before informing Hong Kong.