Chinese health officials said while they could not rule out a limited human-to-human transmission of the H7N9 bird flu, the few cases of "family clusters" did not indicate that the virus could mutate in a way that allows it to spread easily among people.

So far the virus appears to be spreading from infected birds and there was no full-proof evidence that it was being passed on by people, experts from Chinese Center for Disease Control and Prevention said.

Fears about human-to-human infection emerged after a Shanghai family - father and sons - and a couple were confirmed to have been infected despite having no contact with poultry.

But "further investigations are still under way to figure out whether the family cluster involved human-to-human transmission or they were exposed to the same infection source," Feng Zijian, director of the health emergency at the center, told the media in Beijing on Wednesday.

Feng cited as example of a potential infection source the machines Chinese poultry sellers commonly use to remove feathers from chickens. The birds are dipped into hot water tubs that spin at high speeds, and liquid particles containing the virus could be dispersed easily.

"If there is a virus, it can be easily inhaled this way," Feng said. "This is what we suspect to be a major environmental exposure that causes human infections."

Experts have insisted that H7N9's human-to-human infection was rare and could be within a family, as close family members may be all genetically vulnerable to the virus.

The Li family involved two brothers. Their 87-year-old father died of the virus on March 4. The elder son's infection was confirmed in retrospective diagnosis on Monday. The father and brothers were admitted to hospital between February 14 and 24 and all were diagnosed with pneumonia.

The 69-year-old son recovered and was discharged but before that his 55-year-old brother died from pneumonia and respiratory complications.

In the case of the Shanghai couple, the wife was confirmed carrying the virus on April 4, while the husband tested positive for the flu strain on April 11.

Meanwhile, Shanghai reported one new H7N9 case yesterday, while one patient recovered and was discharged from the Shanghai Public Health Center.

So far, Shanghai has reported a total of 32 H7N9 cases, including 11 deaths and five patients that have recovered after treatment. Anther 16 patients are receiving treatment in isolation, said Shanghai Health and Family Planning Commission.

The new case is an 80-year-old woman surnamed Ma. She tested positive for the virus on Wednesday.

Besides Shanghai, Jiangsu and Henan also reported one new case each and Zhejiang reported two new cases, raising the total number of those infected in China to 87, including 17 fatalities.

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