Authorities in Japan stepped up measures to halt A(H1N1) flu, which the World Health Organization declared was not yet a pandemic even as it claimed two more lives in Latin America.

japan close down school swine flu h1n1 pandemic 190509 02 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 A(H1N1) 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 A(H1N1) flu, which has so far infected 173 people in the country.

japan close down school swine flu h1n1 pandemic 190509 01 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, the A(H1N1) flu has killed six people in the United States - although the WHO lists four dead - and one each in Canada and Costa Rica.

New A(H1N1) flu cases have been reported in Chile, Greece, Hong Kong, Japan, Panama, Peru, South Korea and the United States.

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japan close down school swine flu h1n1 pandemic 190509 03 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.

japan close down school swine flu h1n1 pandemic 190509 04 "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 A(H1N1) flu virus, recommending that making seasonal flu vaccines was still the priority.