China has detained at least a dozen people for spreading false rumours about bird flu, police statements showed Wednesday, with authorities seeking to control "panic" as the number of cases rose to 33.

There have been nine deaths since China announced over a week ago that the H7N9 strain of avian influenza had been found in humans for the first time.

Local governments announced five new cases, but state media also reported a four-year-old boy in Shanghai had been released from hospital, the first person to be cured of the H7N9 strain.

Police across the country had held people for spreading "false information" over the Internet about outbreaks of H7N9 where they lived, statements over recent days collated by AFP showed.

The latest such announcement came from the southwest city of Guiyang where three people had been detained for up to 10 days.

Their actions "caused panic among netizens and citizens", local police said.

The boy in Shanghai, whose full name was not given, was diagnosed with H7N9 on April 4, three days after he developed a fever.

Chinese health experts said his recovery showed the benefit of early detection. Doctors said in order to minimise side-effects they did not give him large doses of antiviral drugs, state media reported.

Chinese authorities say they do not know how the virus is spreading though it is believed to be jumping to humans from birds, possibly chickens, pigeons or quail.

The state news agency Xinhua reported that a top laboratory, part of the Chinese Academy of Sciences, had attributed the strain to "wild birds from East Asia and chickens from east China".

The researchers found "no genes in H7N9 were traceable to pigs," it said.

China announced late Wednesday that it had set up a research project to develop a vaccine for H7N9 within the next seven months, according to Xinhua.

The World Health Organization (WHO) said this week that there was no evidence H7N9 was passing from person to person -- a development that has the potential to trigger a pandemic.

Chinese scientists have stepped up monitoring of migratory birds to prevent the virus from spreading that way, state media said.

Another city, Zhenjiang, has banned live poultry sales, following Shanghai and others in Jiangsu province, Xinhua reported.

Shanghai last week suspended trading in live poultry and shut markets in a bid to curb the outbreak, while Hangzhou city culled poultry after finding infected quail.

A Chinese newspaper again raised questions on the delay of more than three weeks between the first victim's death and the announcement by the central government.

The Southern Metropolis Daily claimed testing by Shanghai confirmed H7N9 a week after the man's death and linked the delay to the annual session of China's legislature, when the government seeks to avoid negative news.

"Would not infections and deaths be less (if there had been an earlier announcement)?" asked the newspaper, part of a group known for investigative journalism.

Chinese officials say time was needed to confirm the virus in people for the first time.

And Chinese state media have praised government transparency, saying officials had "learned lessons" from the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak, which Beijing was accused of covering up.

The WHO has also said it is satisfied with China's information sharing for H7N9.

But an academic said public scepticism towards the government in China reflected doubt in a society in which authorities seek to control information.

"What is coming across... is this sense of mistrust of government handling," said David Bandurski of the China Media Project at the University of Hong Kong.

During SARS, rumours of the then mysterious illness and the existence of cases eventually helped force the Chinese government to be more forthcoming about the virus, which killed about 800 people globally.

"Many of the so-called rumours in China turn out to be true," Bandurski said, adding that a lack of official response allowed rumours to spread.