Fears have been raised over the possibility of a strain of bird flu mutating into a virus that could become easily transmissible between humans.

The H7N9 strain of bird flu has seen a surge in cases in China over the past week, with 24 more people suffering symptoms.

The country's public health authorities are on alert for any signals the more widely circulating virus may be adapting or mutating to be easily transmissible between people.

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The new Chinese cases included a 38-year-old man who died on January 9 and a five-year-old girl who is stable in hospital.

Many of the other new patients were either in a serious or critical condition in hospitals, the World Health Organisation (WHO) said.

Several had reported recent exposure to poultry or poultry markets, but the WHO said the source of infections was still under investigation.

The H7N9 bird flu virus emerged last March and has so far infected at least 199 people in China, Taiwan and Hong Kong, killing 52 of them, the WHO said.

Several clusters of cases in people who had close contact with an infected person have been reported in China, but WHO said yesterday that "so far, there is no evidence of sustained human-to-human transmission".

The body said last week that the United Nations health agency had noted the recent rapid increase in human H7N9 infections and was keeping a watchful eye. "So far we haven't seen anything that causes us to change our risk assessment," a spokesman said.

The WHO's assessment is that "the current likelihood of community-level spread is considered to be low".

The Northern Hemisphere's winter flu season is probably largely to blame for a rise in the number of cases of human H7N9 infection in recent weeks after they dwindled to almost nothing last summer.