Japanese cases of the H1N1 strain of influenza reached 135 after the government confirmed 74 new infections, including in a 5-year-old boy and a man his 60s, the Health Ministry said late Monday.

At the annual meeting in Geneva of the World Health Assembly, pandemic preparedness was placed at the top of the agenda. The World Health Organization’s director-general, Margaret Chan, announced the organization was not yet prepared to raise the global pandemic flu alert level to 6, its highest level.

Britain, Japan and other nations urged the organization on the meeting’s opening day to change the way it decides to declare a pandemic — saying the agency must consider how deadly the virus is, not just how fast it is spreading. There has been concern that by raising the global alert level to 5 on April 29, the United Nations health agency had unduly raised alarm.

In her opening address, Ms. Chan said she would take member country’s concerns under consideration. But she warned that the disease’s lethality can vary from country to country, and that while the rapidly spreading virus appeared to be more mild than originally feared — in effect, handing the world a “grace period” — it remained possible that this was “just the calm before the storm.”

“An influenza pandemic is an extreme expression of the need for solidarity before a shared threat,” Ms. Chan said. “We are fortunate that the outbreaks are causing mainly mild cases of illness in these early days.”

She added: “I strongly urge the international community to use this grace period wisely. I strongly urge you to look closely at anything and everything we can do, collectively, to protect developing countries from, once again, bearing the brunt of a global contagion.”

The W.H.O. raised its tally of global confirmed cases of the disease Monday to 8,829 in 40 countries. There have been 74 deaths, 68 of them in Mexico, the apparent epicenter of the global outbreak.

In Japan, authorities ordered more than 1,000 schools to close in and around Kobe and Osaka, cities in western Japan where the infections have been centered. None of the cases has been life-threatening, and there have been no deaths, according to the Health Ministry.

The bulk of the infections have been found among high school students in the region who have no record of recent overseas travel, which health officials say signals the start of a wider outbreak.

A W.H.O. spokesman, Dick Thompson, said there was still not enough evidence to conclude that the disease was spreading in a sustained way outside of North America, which would be required for the organization to raise its pandemic alert level to 6.

All of the confirmed cases in Japan, Mr. Thompson said, derived from infection clusters within schools in Kobe and one in Osaka, and from the family members of people infected in those schools. Sustained community-level transmission, he said, requires cases of the virus to begin to erupt spontaneously, with no apparent link to other sufferers, over a period of time.

“We don’t see sustained community spread at this moment,” Mr. Thompson said, referring to the Japanese cases.

Japan is well known in public health circles for being exceptionally nervous about flu. Masks are common on subways because it is considered rude to go out in public without if you are sneezing.

Over the weekend, officials in white masks scrambled to set up makeshift tents and telephone hotlines to handle the surge in suspected cases. Osaka city said its public schools would remain closed for seven days, while Kobe imposed similar school shutdowns.

Governor Toshizo Ido of Hyogo prefecture, the region surrounding Kobe, said he would ask the central government to grant financial aid to the affected localities, including support for business owners affected in the outbreak.

In Tokyo, Prime Minister Taro Aso called for calm. “Many infected people received the right treatment at an early stage, and have recovered,” Mr. Aso said at a government task force meeting. “I would like everybody in Japan to act calmly.”

Until Friday, Japanese officials thought they had averted the virus after a string of suspected cases among people entering Japan proved negative. Japan has been sending medical examiners to each flight arriving from North America to take the temperatures of those on board.

On Friday, four Japanese returning from Canada were found with the H1N1 strain, and quarantined together with 50 other passengers. But on Saturday, the authorities confirmed that a 17-year-old student in Kobe with no recent overseas travel had been infected.

Since then, new cases have been reported in Kobe and neighboring Osaka. There have been no cases in Tokyo.

Still, the rapid spread of the disease has come as a shock to Japan, which drew up an extensive action plan to contain influenza after bird flu appeared in Asia in 2004. Turkey, India and Chile also reported their first swine flu cases over the weekend. Late Sunday night, Hong Kong confirmed its third case of swine flu, a 23-year-old man from southern China who had been studying in the United States. He arrived in Hong Kong on Saturday evening on Cathay Pacific Flight 831 from New York.

A spokesman for the Hong Kong health department said the man developed a fever during the flight and was spotted by a thermal scanner at the airport when he left the plane. Authorities put him in an isolation ward at Princess Margaret Hospital and issued a call for other passengers on the flight to report for testing. As of Sunday night, the spokesman said, 22 passengers and a crew member had been quarantined at a camp in rural Hong Kong.

Hong Kong’s first case of flu, on May 1, resulted in the quarantine of hundreds of travelers and hotel guests. The second case occurred last week.

WHO Director-General Margaret Chan said the flu outbreak needed to be tackled with the utmost seriousness because there were still many unknowns about which path it will take.

“We are all under pressure to make urgent and far-reaching decisions in an atmosphere of considerable scientific uncertainty,” she told her U.N. agency’s World Health Assembly.

“For the first time in humanity, we are seeing, or we may be seeing, pandemic influenza evolving in front of our eyes,” Chan said, noting this had not been the case in 1968, 1957 or 1918.

Richard Besser, head of the U.S. Centers for Disease Control and Prevention (CDC), told a high-level meeting during the week-long Assembly that the new H1N1 virus had spread to nearly all 50 U.S. states and showed no signs of abating.

“While we are not seeing the seriousness of illness that was initially reported in Mexico, the outbreak is not over,” he said, adding the new virus was likely to circulate worldwide.

Assembly delegates were to discuss how best to harness drugs and vaccines and respond to the H1N1 flu, which has caused mild symptoms in most of the 8,829 patients infected to date in 40 countries, killing 74 people.

FLEXIBILITY

Mexico’s Health Minister Jose Angel Cordoba told the high-level segment that 3,646 cases had been confirmed in Mexico where there was a slow but steady fall in the number of cases.

Officials will also seek an agreement on how samples of the virus should be handled and shared with pharmaceutical companies working to develop vaccines to fight the strain, which is a genetic mixture of swine, bird and human viruses.

Rich and poor countries remain at odds over whether the biological material can be patented. The meeting will also discuss poor countries’ needs for antiviral drugs like Roche’s Tamiflu and GlaxoSmithKline’s Relenza and any vaccines developed to confront the strain.

Egypt’s envoy said that the threat from the deadly bird flu virus should not be ignored, noting there had been three new human cases of H5N1 in his country in the last week.

Chan said the new strain might pose particular risks when it mixed with the H5N1 flu virus, which has proven deadly in birds and humans but not transmitted easily between people.

It could also cause risks for people with HIV/AIDS and tuberculosis, and in crowded shantytowns, she said.

Chan raised the WHO’s pandemic alert level to Phase 5 last month in response to the spread of H1N1 in North America, which has had 95 percent of the nearly 9,000 confirmed cases to date.

Under the U.N. agency’s rules, signs that the disease is spreading in a sustained way in a second region of the world would prompt a declaration that a full pandemic is underway.

Countries from Britain to China said the WHO should have more flexibility to interpret its pandemic alert scale and decide whether to declare a full pandemic. Declaring a top-level phase 6 alert should reflect the severity of a new virus, not just its geographical spread, they said.

WHO laboratories have confirmed 125 cases in Japan, making it the largest cluster of H1N1 outside of the Americas, followed by Spain with 103 and Britain with 101, a top WHO official said.

Most of those cases have been deemed related to travel or restricted within schools, and not examples of the new virus spreading freely in broad communities, according to the WHO.

Chan and U.N. Secretary-General Ban Ki-moon meet top pharmaceutical executives on Tuesday to discuss their ability to make vaccines to fight the H1N1 strain. Making a pandemic shot could lead to lower output of vaccines for seasonal flu, which kills up to 500,000 people a year.

The government ordered schools closed in parts of the port city of Kobe, where the Ministry of Health said a male high school student tested positive for the same strain that has killed more than 70 people worldwide, most of them in Mexico. Unlike the other cases confirmed in Japan so far, the student had not traveled abroad recently.

Two other students at the same school who also haven’t left the country recently are also suspected of having the virus. They tested positive in tests done by the local government, but authorities were waiting for results from a national lab.

All three were recovering at medical facilities in Kobe, which is about 270 miles west of Tokyo. None of their names were released.

The confirmed case is Japan’s fifth overall. The first four cases, three high school students and a teacher who had recently returned from a school trip to Canada, were detected as they entered the country at Narita Airport, Tokyo’s main international hub. The health ministry did not release the location of their school.

The H1N1 swine flu virus is a new influenza strain for which people have no immunity. Health officials have warned that it could spark a pandemic, infecting millions of people.

Japan has until now sought to take advantage of its lack of land borders to stop the virus from entering the country.

Medical staff were sent to all of its international airports, where doctors in masks and biohazard suits conducted tests of passengers before they even left their planes.

But experts have noted that this flu seems to have a long incubation period — five to seven days before people notice symptoms.

Health Minister Yoichi Masuzoe said a health official had been dispatched from Tokyo to help investigate how the disease had spread to the student, who still has a cough and sore throat but is recovering.

Masuzoe said in a national television broadcast that schools in Kobe would be closed for seven days to try to prevent further spread. He urged people to remain calm.

“The most important thing at this stage is for citizens to act calmly based on accurate information,” he said.

According to the World Health Organization, some 7,520 people in more than 30 countries have been sickened by the virus, mostly in the U.S. and Mexico.