As the agency opened its annual meeting, WHO Director-General Dr. Margaret Chan said she had listened carefully to the concerns of member states and would follow their instructions.
Britain's Health Secretary Alan Johnson warned that raising the flu alert to phase 6 - the highest level - could needlessly trigger costly and potentially risky actions, such as a switch from seasonal to pandemic vaccine, even though the virus so far appears to be mild.
"We need to give you and your team more flexibility as to whether we move to phase 6," Johnson said.
Right now the alert level is at phase 5, meaning a global outbreak is "imminent."
Chan repeated her warning that the new swine flu virus continues spreading rapidly and could pose a grave threat to humanity even though the fatality rate was low, with 76 deaths out of 8,829 cases and no major outbreaks outside North America.
"A new influenza virus with great pandemic potential, the new influenza A (H1N1) strain, has emerged," she said. "This virus may have given us a grace period, but we do not know how long this grace period will last," Chan said. "No one can say whether this is just the calm before the storm."
A pandemic announcement would likely have severe economic consequences: it could trigger expensive trade and travel restrictions like border closures, airport screenings and quarantines, as countries not yet affected struggle to keep the virus out.
Governments may also fear outbreaks of mass panic, social disruption and increased pressures on their health systems. Under public pressure, extraordinary measures such as large-scale pig slaughters like the recent one in Egypt could be taken, whether or not they are scientifically justified.
Among the countries urging WHO to reconsider its pandemic scale was Mexico, which has suffered the most deaths and virtually shut down its economy for several days in response to the outbreak.
"People don't understand what 4, 5 or 6 means," Mexico's Health Minister Jose Angel Cordova told reporters. "They think that when you go to a higher level things are worse."
"Flexibility will very much help the economy," he added.
U.S. Health and Human Services Secretary Kathleen Sebelius told The Associated Press she wanted more information on the proposal before taking a position, but that she was impressed how many countries supported it.
Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, told the WHO meeting that the outbreak is "not winding down" in the United States and "widespread transmission" continues. He also said the epidemic was not over in Mexico.
Speaking a day after New York school assistant principal Mitchell Wiener died of swine flu, Besser said the world needed to maintain its vigilance against the virus.
At least 40 countries have now confirmed cases, with Japan reporting the largest jump to over 130 in the space of four days. Most of the new cases involved high school students in the western prefectures of Hyogo and Osaka who had not traveled overseas.
Spain and Britain have the highest numbers of cases in Europe, reporting 103 and 101 cases, respectively.
Britain's health secretary said that while the country was concerned about the outbreak, it was still far below the number one would expect for seasonal influenza.
"We could have declared a pandemic in order to enhance the protection to our people from flu, and because we've done that we could cease further production of seasonal flu vaccine," Johnson told The AP.
Seasonal flu kills hundreds of thousands of people worldwide each year. Stopping vaccine production for that strain could actually lead to more deaths that would occur from swine flu, he said.
Johnson said considerations other than just the geographic spread of the virus were important.
"Severity is a part of it," he said.
"It's certainly something we will look at very closely," Dr. Keiji Fukuda, WHO's flu chief, said of the proposal to include other factors when determining the alert level.
So far the virus appears to be mild, though scientists are concerned that many of the more severe symptoms have turned up in younger people. Flu is normally most dangerous to babies and the elderly.
Chan also noted that the disease could combine with other flu strains currently circulating around the world, including the lethal H5N1 bird flu virus that has so far struggled to transmit among humans.
The signal for starting pandemic vaccine production has yet to be given, but it is essential that countries use their stockpiles of drugs wisely, she said.
"Manufacturing capacity for antiviral drugs and influenza vaccines is finite and insufficient for a world with 6.8 billion inhabitants," Chan said. "It is absolutely essential that countries do not squander these precious resources through poorly targeted measures."
Chan and U.N. Secretary-General Ban Ki-moon are scheduled to meet with senior representatives of pharmaceutical companies Tuesday to discuss the vaccine question. The U.N. declined to name the companies but major vaccine producers include Sanofi-Aventis, GlaxoSmithKline and Baxter International.
Key questions that need to be answered include: how soon can a pandemic vaccine be produced, and how much of it will be available to each country. Many governments, including Britain, have already signed large advance orders, potentially depriving poorer countries of a chance to buy their own stock.
Johnson said his proposal for WHO to consider carefully the impact before moving to phase 6 was not an attempt at interfering with the global body's decision-making powers. The choice on moving to the highest level would remain with WHO and its medical experts, he said.
"A scientific argument put to politicians is a very effective argument."