Japanese authorities have taken measures to tackle the swine flu menace on a war-footing even as the WHO revealed that it was not according a pandemic status to the infection. Meanwhile the flu claimed two more lives in Latin America.
WHO chief Margaret Chan told member states at the opening of the UN health body's annual assembly they may be facing a "calm before the storm" but there was no reason to raise the alert level to its maximum six.
The current level of five indicates a pandemic is imminent, although swine flu is now present in 40 nations, infecting nearly 8,300 people and causing 74 deaths.
"We need to warn the public whenever necessary, but reassure them whenever possible. This is a difficult balancing act," Chan told the WHO meeting.
Those WHO figures do not include two more deaths reported Monday in Mexico, epicentre of the influenza A(H1N1) virus outbreak but where the disease is now waning.
In Japan, authorities ordered more than 4,000 school and kindergartens to shut, double the previous day, to slow the spread of swine flu, which has so far infected 173 people in the country.
Many people in the affected urban areas were wearing face masks after the western cities of Kobe and Osaka became the first in Japan to suffer domestic outbreaks of the virus, which spread rapidly through two schools.
The virus is believed to have spread after high schools from the two cities played in a volleyball tournament, and some players and coaches felt feverish afterwards.
Experts warn the virus will likely soon spread to other regions, including Tokyo, which with almost 36 million people is the world's most populous urban area and the heart of the Japanese economy.
"The virus's spread to Tokyo is near certain and it would be little wonder if the virus had already landed in Tokyo undetected," said Yukihiro Nishiyama, virologist at the medical department of Nagoya University in central Japan.
Apart from Mexico, swine flu has killed six people in the United States, although the WHO lists four dead, and one each in Canada and Costa Rica.
New swine flu cases have been reported in Chile, Greece, Hong Kong, Japan, Panama, Peru, South Korea and the United States.
In New York, 12 schools have been temporarily closed following the death of a 55-year-old school assistant principal.
Separately, the US government reported Monday more than 400 new cases, to take the nation's total number of infections above 5,000.
"We cannot stop the virus from spreading. We should not be surprised to see more cases," New York Mayor Michael Bloomberg warned.
Meanwhile Canada ended a recommendation to postpone "non-essential" travel to Mexico, saying the risk of contracting A(H1N1) had levelled off.
In Geneva, Chan acknowledged scientific uncertainty surrounding the virus and the need for more information.
"We do not know how long this period will take, if this is the calm before the storm," Chan told the assembly, adding that there was "every reason to be concerned with the interaction with other viruses."
Health authorities have said they expect the number of A(H1N1) flu cases to rise in the southern hemisphere over the next months as the region enters its autumn and winter seasons.
The WHO also advised the pharmaceutical industry not to switch the focus of its production onto the new swine flu virus, recommending that making seasonal flu vaccines was still the priority.