The World Health Organisation's annual assembly opened on Monday. Due to the sharp spike in swine flu infections in Japan, the spectre of a global flu pandemic hung over the gathering.
"We are all under pressure to make urgent and far reaching decisions in an atmosphere of considerable scientific uncertainty," WHO Director General Margaret Chan told the 193 member states in a special debate on the new influenza A(H1N1) virus.
However, Chan, who would have the power to declare a pandemic after consulting a panel of scientists, said more information was needed on the virus that emerged in Mexico and the United States a few weeks ago.
"We have lived for five long years under the threat of pandemic caused by the lethal H5N1 avian influenza virus. This has left our world very prepared but also very scared," said Chan.
"We need to warn the public whenever necessary but reassure them whenever possible. This is a difficult balancing act," she explained.
Since the swine flu outbreak was signalled by Mexico last month, the WHO has raised the global flu alert to level five, one step short of a pandemic.
More than 8,800 people have been infected by the new virus after it spread to 39 other countries with travellers, according to the WHO.
But the UN health agency was keeping a close watch on the growing number of cases in Japan, where more than 2,000 schools were closed Monday in a bid to slow the spread of the virus.
Some 129 people have been infected according to Japanese authorities, who only days ago had detected just four cases in travellers from the United States.
WHO officials have warned that sustained transmission in a community outside the Americas, without a direct link to travellers, would be reason enough to declare a pandemic, alert level six, marking the global spread of the virus.
However, they have also emphasised that a pandemic denotes geographical spread of the virus, not the severity of its symptoms.
Over the weekend, a renowned European virologist urged watchdogs to keep up their guard, even if the new strain of H1N1 virus had turned out to be less worrisome than thought a few weeks ago.
"The current H1N1 threat is serious," Dutch professor Albert Osterhaus said on the sidelines of a congress in Helsinki.
Member states decided to shorten the assembly to five days instead of 10 so that officials can focus on national preparations against flu.
Discussions on faltering WHO-led attempts to eradicate polio and curb malaria, or health care in poor countries, have been pushed down the agenda, while debates on advancing tuberculosis, viral hepatitis, or rules on human organ transplants could be postponed for a year.
Mexico's Health Minister Jose Angel Cordova symbolically handed over to Chan the scientific details of the A(H1N1) virus that will allow development of a vaccine, just before the assembly began.
UN Secretary General Ban Ki-moon was set to meet pharmaceutical companies here on Tuesday to discuss production of a pandemic flu vaccine, which could take up to six months to develop.
The WHO is still considering whether to take the risk of halting seasonal flu vaccine output in order to free up production capacity for large scale pandemic vaccine production.
Influenza and pandemic preparations have been a regular topic at the meeting in recent years after H5N1 bird flu took hold in humans.
At a meeting in Geneva ahead of the assembly, governments failed to conclude deal on the sharing of virus research material and vaccines in case of a global pandemic, after years of wrangling.
Some Asian nations hit by bird flu want guarantees that populations of poor countries will have access to commercially-produced vaccines made from the samples they supply.