In a report released on Thursday, the WHO said bird flu viruses were evolving and a global pandemic could not be ruled out.
"The viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat," the UN health agency said.
"We don't know whether the pandemic will occur next week or next year," said Dr Klaus Stohr, the WHO's influenza chief, who was speaking in Geneva on the findings of a conference earlier this month in Manila, Philippines.
The H5N1 strain of bird flu in southeast Asia has jumped from animals to humans, but not from person to person.
It has killed 36 people in Vietnam, 12 in Thailand and four from Cambodia.
"The little information that we have could possibly identify that the virus is changing and the way it interacts with humans is changing," Stohr told reporters.
"If public health authorities move too soon, then unnecessary and costly actions may be taken," he said.
But if action is delayed until there is unmistakable evidence that the virus has become sufficiently transmissible among people, then it may be too late for an effective response, the WHO has warned.