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.