BEIJING - China and the World Health Organization (WHO) said on Wednesday that all the H7N9 infections identified so far are isolated cases and there is no evidence of any inter-human transmission.

Although several cases of family-clustered infections with H7N9 avian influenza have occurred in China, there is no definitive evidence indicating that the virus was transmitted from the same source or among different people, according to a China-WHO joint statement.

Moreover, there has been no sustained human-to-human spread monitored so far, the statement said.

However, it added, the possibility of such situation can not be ruled out, as scientists are not sure whether the virus will develop to be inter-human communicable.

Severe symptoms have been observed in most of the infection cases, but uncertainty remains over how many people may be carrying the virus without showing any symptoms, the statement said.

"That is why we are carefully working on the virus and closely monitoring its spread," it explained.

Also on Wednesday, Liang Wannian, a senior official with China's National Health and Family Planning Commission, warned that the disease may further spread.

Infection cases have been detected in seven of the country's provincial-level regions, and such infections may happen in more places in China before the source of the disease is clearly identified and effectively controlled, Liang said.

The authorities will intensify the monitoring of infections among animals and people, the official added.

At a China-WHO joint press conference on Wednesday, Keiji Fukuda, WHO assistant director general for health security and environment, hailed the Chinese government's efforts in dealing with H7N9.

He expressed his appreciation for the work of China's disease prevention and control institutions and local governments in their quick response and sharing of key information regarding the virus, disclosure which has been very useful in making judgements and will expedite future possible vaccine production if needed.