The WHO wants scientists to help clear up the criteria needed for declaring a pandemic as swine flu cases worldwide soared to nearly 13,000 Tuesday, spreading further in the Middle East.
"We are trying to see what kind of adjustments must be made to make sure that the definitions really meet the situation," said World Health Organisation interim Assistant Director General Keiji Fukuda.
The move follows appeals by several countries for more caution before moving up a step from the current phase five alert to declaring a pandemic for new A(H1N1) virus.
The virus has caused 92 deaths and infected 12,954 people in 46 countries since it was first uncovered last month, according to the WHO's latest figures.
The Gulf kingdom of Bahrain reported its first confirmed case of A(H1N1) swine flu Tuesday, the second state in the Gulf region hit with the disease.
"The first case is a young Bahraini man of 21 who has returned from studying in New York," health ministry spokesman Adel Abdullah told AFP.
Britain revealed an outbreak of 47 new cases nearly all of them linked to a school in the West Midlands of England, while Greece reported its second flu victim, a 21-year-old man.
Other new cases surfaced Tuesday in Asia with South Korea now having 27 people infected and Hong Kong reporting its 10th victim.
For travelers, the spread of swine flu around the globe has disrupted many travel plans.
More than 2,000 passengers on a cruise ship in Australia found themselves grounded and asked to quarantine themselves for a week after eight passengers and a crew member tested positive for the A(H1N1) virus.
"We are all requesting that all people onboard this ship remain in quarantine for seven days," New South Wales chief health officer Kerry Chant told reporters after the Pacific Dawn ended its South Pacific cruise in Sydney.
That delayed the ship's next departure for the Great Barrier Reef, but holiday-makers seemed to take the inconvenience in stride.
"We'd be complaining if we all got sick, and the recycling air conditioning doesn't help flu and all the rest of it," one passenger told the Australian Broadcasting Corporation.
In Russia, the mere suspicion of swine flu sent passengers back home.
A US charter plane with five passengers and two crew turned back from a remote airport in eastern Russia after local medical officials said they suspected a case of swine flu on board, RIA Novosti reported on Tuesday.
"During a sanitary check in the cabin of the plane... doctors found one of the two crew members had an elevated temperature," a spokesman for the local branch of Russia's trade and sanitation watchdog told the agency.
The plane's pilot decided to turn around and fly back to the United States rather than submit to medical examinations locally.
Tuesday's WHO figures show the biggest growth in swine flu cases in the United States, with 212 more people infected, and one more death, bringing the US total to 6,764 and 12 deaths.
The UN health agency wants to look more closely at how and when it should declare a flu outbreak as a pandemic, a phase six alert.
Fukuda admitted it was quite possible the A(H1N1) influenza would continue to spread to other countries and would probably then be considered a pandemic.
"But right now we're clearly in the early times of the evolution of this virus and we'll see where it goes," he added.
Under the current rulebook, phase six would involve a purely geographical step with sustained community spread in another WHO region to the Americas, where the virus first emerged in April.
But officials said last week they were also taking into account issues such as the severity of the virus, possible changes in the pattern of illness, its impact on poor countries or circulation in the southern hemisphere where it could mix with seasonal flu.