SHANGHAI (AFP) -- A World Health Organisation official reiterated Monday there is still no evidence that a new strain of deadly bird flu is passing in a "sustained" fashion from person to person, despite fears some family members may have infected one another.

"Right now we do not see evidence of sustained human-to-human transmission", Keiji Fukuda, the WHO's assistant director-general for health security and environment, said at a press conference.

He added, however, that officials "are always worried whether there could be person-to-person transmission".

Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have so far declined to put it down to human-to-human transmission.

Commenting on the clusters, Fukuda said that, based on available evidence, "it is not clear why we have these cases".

He said families where more than one person has contracted the virus may have caught it from animals, the environment or one another.

Health experts differentiate between "sustained" human-to-human transmission and cases in which family members or medical personnel caring for the ill become infected.

Fukuda spoke as a WHO team wrapped up a visit to Shanghai, the centre of the country's bird flu outbreak that has killed 20 people, as part of an investigation into how the H7N9 virus is spreading.

Since announcing on March 31 that the virus had been discovered in humans for the first time, China's health ministry on Sunday confirmed a total of 102 cases, in Shanghai, the capital Beijing and four provinces.

"There has been no discovery of evidence of human-to-human transmission," the ministry said in a statement.

Experts fear the prospect of such a virus mutating into a form easily transmissible between humans, which could then have the potential to trigger a pandemic.

The WHO's representative in China, Michael O'Leary, said Friday that the purpose of the 15-member team's week-long visit was to study whether H7N9 was spreading among humans.

"The primary focus of the investigation is to determine whether this is in fact spreading at a lower level among humans. But there is no evidence for that so far except in these very rare instances," O'Leary said.

The son of a man who was Shanghai's first case of H7N9 was confirmed to have contracted the virus after an initial test ruled it out, Chinese officials said last week.

The Shanghai government also said the husband of a woman confirmed with the virus had become sick with H7N9, but added there was not enough evidence to verify transmission between them.

"Family clusters in general do not change our understanding of the characteristics of the disease," said Feng Zijian, an official of China's disease control centre.

"It is still passed from poultry to people and there is no evidence of human-to-human transmission," he said on Wednesday.