Since the H1N1 flu first emerged in Mexico and the US in April, it has spread to 74 countries around the world.
The latest tally by the WHO records 27,737 cases. Most cases have been mild and required no treatment but 141 people have died.
Thursday's meeting comes as Australia announced that five people had been admitted to hospital intensive care units in recent days, "pretty ill" with the virus according to health officials.
Australia now has more than 1,200 cases of the illness but has had no deaths.
The WHO pandemic alert has been at phase 5 since May 1, meaning it thought a global outbreak was imminent.
Moving to phase 6, the highest level, would acknowledge that a pandemic had begun, obliging drug companies to fast-track production of a swine flu vaccine.
It could also put pressure on countries to activate their own pandemic preparedness plans if they had not already done so, possibly devoting more money to health services or imposing measures such as quarantines, school closures, travel bans and trade restrictions.
Questions over delay
But the declaration of a pandemic would also likely trigger fresh questions about why the step was delayed for weeks as the virus continued to spread.
According to WHO's pandemic criteria, a global outbreak means a new flu virus is spreading in at least two world regions.
|Australia has recorded more than 1,200 confirmed cases of the flu [GALLO/GETTY]
But with thousands of cases in North America, more than a thousand in Australia and hundreds in Japan and Europe, many experts say that threshold has already been reached and that the UN agency has held off on making the pandemic call for political reasons.
Michael Osterholm, a flu expert at the University of Minnesota who has advised the US government on pandemic preparations, said "if you look at the science, we were at phase 6 weeks ago".
"What's happening right now is not about public health surveillance and science - it's about politics and risk communication," he told The Associated Press on Wednesday.
Osterholm said WHO's delayed decision has cost the agency credibility.
"As soon as you try to incorporate risk messaging into science, you are on a slippery slope," he said. "WHO has exacerbated the issue by dancing around it."
One flu expert said WHO's pandemic declaration would mean little in terms of how countries were responding to the outbreak.
"The writing has been on the wall for weeks," said Chris Smith, a flu virologist at Cambridge University, adding he did not know why WHO had waited so long to declare a pandemic.
"WHO probably doesn't want people to panic, but the virus is now unstoppable."
In May, several countries urged WHO not to declare a pandemic, fearing it would spark mass panic.
The agency appeared to cave in to the requests, saying it would rewrite its definition of a global outbreak so that it would not have to declare one right away for H1N1.
But WHO officials have been concerned in recent days after seeing media reports and health experts discussing more cases than were being reported by the countries themselves.
WHO said declaring a pandemic would not mean the situation was worsening, since no mutations have been detected in the virus to show it is getting more deadly.