Two United Nations agencies are urging Vietnam to be vigilant in its fight against bird flu after three new human cases were discovered this year, including one woman who died.

The World Health Organization and the U.N.'s Food and Agriculture Organization warned Vietnam to monitor its situation closely.

Vietnam has seen more bird flu deaths than any other country except Indonesia.

In a joint statement Tuesday, the WHO's Vietnam-based representative, Jean-Marc Olive, said experience has shown that the threat is highest at this time of the year.

The virus normally passes from bird to human, but experts fear it could begin spreading from one person to another and start a pandemic that could kill millions of people.

Olive said there is a real possibility of human-to-human transmission of the disease.

The U.N. says Vietnam has already seen 52 bird flu deaths since 2003. Indonesia has seen 115 deaths.

On Saturday, a woman died of the deadly H5N1 strain of the virus in Vietnam's first fatality from the disease this year.

Some information for this report was provided by AFP and Reuters.