Three members of the same Chinese family have contracted H7N9 bird flu in the province worst-affected by the current spike in cases, the official Xinhua news agency reported.

A couple and their daughter in Hangzhou, the capital of the eastern province of Zhejiang, were infected one after another, Xinhua said late Tuesday, without giving further details.

Health authorities are investigating, it added.

China's human H7N9 outbreak began in February 2013 and sparked fears the virus could mutate to become easily transmissible between people, potentially triggering a pandemic.

Both Chinese authorities and the World Health Organization (WHO) have said there has been no evidence so far of sustained human-to-human transmission.

But limited spread, such as between relatives in close contact, is possible, and there have been previous such family clusters.

So far this year, China has confirmed 110 human H7N9 cases, including 22 deaths, according to an AFP tally of reports by local authorities.

By comparison there were 144 infections and 46 deaths in all of 2013, according to official figures.

The WHO said on Wednesday that the spike in cases this year was not surprising due to seasonal factors, rather than a virus mutation.

"Today there is no evidence that the characteristics of the virus have changed in a way that would explain an increase in cases and change in case fatality," WHO Representative in China Bernhard Schwartländer told AFP in an email.

Zhejiang alone has seen 53 cases this year, almost half the national total, and 12 deaths.

Local authorities are to permanently ban live poultry markets in major cities, according to reports in official media.