The World Health Organization has warned that it expects to see more sporadic cases of human bird flu in China, and potentially abroad, and said public vigilance is needed to contain the virus.

The alert comes as authorities in Zhongshan, Guangdong province, confirmed two new cases of H7N9 avian flu on Wednesday. Both patients were in critical condition after being exposed at live poultry markets.

"Given the increasing number of human cases and constantly evolving nature of influenza viruses, continued vigilance and close monitoring are needed to detect infections," the WHO China Office said in a statement.

Winter is the peak season in China for zoonotic flu — strains that can pass from animal to human — and so far several cases of H7N9 have been reported in regions including Shanghai and Fujian and Anhui provinces. Two people died in Anhui in December.

Most of the recent cases involved exposure to infected live poultry or contaminated environments, such as live poultry markets, according to the WHO.

"The information to date does not support sustained human-to-human transmission, although limited human-to-human transmission cannot be excluded," it said. "We expect to continue to see a number of sporadic cases."

China has culled more than 170,000 chickens in four provinces since October and closed some live poultry markets, Reuters reported.

H7N9 was first reported in China in spring 2013. According to WHO data, infections were detected in two travelers arriving in Canada from China in 2015 and one in Malaysia in 2014.

"It's possible similar cases will be detected in other countries among travelers from affected areas, although community-level spread in other countries is unlikely," the statement said, adding that the WHO "does not advise special screening at points of entry" regarding avian flu, "nor does it currently recommend any travel or trade restrictions".

He Xiong, deputy director of the Beijing Center for Disease Control and Prevention, said he sees no possibility that recent cases will spark an epidemic, since there is no evidence the flu can be transmitted between humans.

The WHO advised, "It's important the general public continue to take precautionary measures to reduce exposure to avian influenza viruses from infected poultry." Such measures include avoiding direct contact with live poultry and adopting good hygiene practices while handling, slaughtering and preparing poultry for consumption.

Mao Qun'an, a spokesman for the National Health and Family Planning Commission, said in December that China "is well prepared to contain outbreaks".