The current outbreak of a virulent strain of bird flu in Vietnam poses a big risk to public health, says the new head of the World Health Organization.

Dr. Margaret Chan, who holds Canadian citizenship and assumed the post on Thursday, urged the world to remain vigilant against the threat posed by the H5N1 strain of bird flu.

"We owe it to the community we are serving to keep our watch," Chan said Thursday from WHO headquarters in Geneva. "The risk is there. We should not let our guard down."

Complacency "is our biggest enemy" in preparing for the next influenza pandemic, she said, but added that panic was not appropriate either.

The number of bird flu cases has increased in recent weeks, according to WHO.

Vietnam faces an outbreak on a poultry farm in the Mekong Delta provinces, the first in that country in almost a year. Bird flu has killed 42 of the 93 people infected in Vietnam.

The human death toll from H5N1 is highest in Indonesia with 57, WHO figures show.

Chan, 59, holds Chinese and Canadian citizenship. She received her medical degree from the University of Western Ontario in London.

China nominated her for WHO's top job, and she is the first Chinese citizen to hold such a high-ranking UN position.

Chan's predecessor as WHO chief, South Korea's Lee Jong-Wook, died suddenly in office last year.

With files from the Canadian Press