Investigations in Vietnam earlier this year concluded that "the viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat," the U.N. health agency said.
"There is no evidence in any direction, but there are concerns," said Dr. Klaus Stohr, WHO's influenza chief, who was speaking in Geneva on the findings of a conference earlier this month in Manila, Philippines. "There are elements that are worrisome."
The H5N1 strain of bird flu in Southeast Asia has so far only 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.
Monitoring of the virus should be stepped up and there should be increased control of the disease in poultry, WHO said in a report on its Web site.
But evolution of a pandemic strain of virus can happen in numerous small steps, none of which is sufficient to signal clearly that a pandemic is about to start, WHO cautioned.
"If public health authorities move too soon, then unnecessary and costly actions may be taken," WHO 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.
"This poses a difficult public health dilemma," WHO said.