WHO Director-General Lee Jong-wook on Tuesday warned Asian health ministers meeting in Colombo, Sri Lanka, that there was a fresh threat of the virus spreading quickly.
"He cautioned them about a possible impending Avian influenza pandemic," the Geneva-based organisation said in a statement after the two-day meeting.
"Dr Lee said that the reservoir of the virus had moved from domestic poultry to ducks and has now been established in migratory birds in China.
"The danger of the expanding geographical range of the virus increases possibility for human cases to occur, increasing the potential for it to become more contagious."
The organisation said Asian countries at risk needed to improve efforts at case detection to ensure an early warning to others, mobilise international stockpiles of antiviral drugs and speed up vaccine manufacturing capacity.
Other experts have previously said wild birds were spreading avian influenza, known as H5N1, far beyond its epicentre in the backyard farms of Asia.
But scientists have argued over the disease transmission path, which most recently saw it spread to parts of Russia and Kazakhstan and prompted warnings it could next move into Europe and South Asia.
Bird flu has killed 62 people in
Asia in the past two years
Two Asia-based British ornithologists this week contested the theory that the virus was spread by migratory birds.
Martin Williams, an ornithologist in Hong Kong, and fellow bird watcher Nial Moores in South Korea say their studies show the spread of bird flu does not match with migration patterns of wild birds in Asia.
Asia has been battling bird flu since late 2003, with vaccination campaigns and massive culls of tens of millions of chickens and ducks that have devastated poultry industries, particularly in Thailand and Vietnam.
Bird flu has killed 62 people in Asia in the past two years, including 43 in Vietnam.