Among the new cases, reported from several different provinces of China, was a 38-year-old man who died on Jan. 9 and a five-year-old girl from Guangdong province who became ill on Jan. 14 and is now stable in the hospital.

Many of the other new patients were either in a serious or critical condition in hospitals, the 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 in March last year and has so far infected at least 199 people in China, Taiwan and Hong Kong, killing 52 of them, according to an update from the WHO's spokesman Gregory Hartl.

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

Hartl told Reuters 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,” he said.

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

Experts say the Northern Hemisphere's winter flu season is probably largely to blame for a significant increase in the number of cases of human H7N9 infection in recent weeks after they dwindled to almost nothing in July and August of 2013.

But they have cautioned that public health authorities and doctors must be on alert for any signals that the more widely circulating virus might be adapting or mutating to become easily transmissible between people.