"We expect to continue to see a small number of sporadic cases. To date, there is no evidence of sustained human-to-human transmission," said Dr Bernhard Schwartlander, the WHO representative in China.

Schwartlander said whether the H7N9 virus could actually cause a pandemic was unknown, although "in principle, [it] carries a risk".

The third new human H7N9 bird flu case, a three-year-old boy in Dongguan who was confirmed as having contracted the virus on Tuesday, was in a stable condition and expected to be discharged from hospital soon. He had not developed a fever.

The child had gone to a wet market with poultry stalls, but did not have contact with the fowl.

Sixty-three people who were in close contact with him have been put in quarantine for a week. To date, the mainland has confirmed 138 human cases of H7N9 bird flu, 45 of them fatal.

Ben Cowling, associate professor at the University of Hong Kong's school of public health, said it was too early to say if there would be a major outbreak during the flu season this winter.

Cowling has published a paper in the medical journal stating that H7N9 may follow a similar seasonal pattern to H5N1 bird flu, and predicted it would reappear this autumn and potentially cause a major epidemic this winter.

Professor Xu Xiaoyuan of Peking University's department of infectious diseases, said no significant genetic mutation had been detected in the virus, meaning there was a very low chance of a community-wide outbreak or human-to-human transmission. He said it was possible there would be sporadic cases in the winter flu season.

Professor Yuen Kwok-yung, a microbiologist at the University of Hong Kong, said imported H7N9 cases were possible because two Hong Kong residents had previously developed H5N1 infection after visiting wet markets on the mainland.

He said the possibility of a human or poultry outbreak in Hong Kong was not high, as all imported poultry was sampled and tested.