The number of (A)H1N1 cases has risen rapidly in the urban areas of Kobe and Osaka since the first confirmed domestic infection -- a 17-yAuthorities said the real number of infections could already be in the hundreds with the virus spreading fast in the densely populated island nation.

"This is not just a matter for Hyogo and Osaka (prefectures)," said Health Minister Yoichi Masuzoe. "We have to act swiftly by assuming the virus is already widespread across the country."

The Japanese outbreak increases the chance the World Health Organization may raise its pandemic alert level to six -- the top level which would indicate sustained community transmission in a second region outside the Americas.
Experts in Japan now believe the virus spread quickly between the two urban areas after high schools from Kobe and Osaka met for a volleyball tournament, with some players and coaches feeling feverish after the games.
"We have to contain this in the region, but it is becoming very difficult," said Toshizo Ido, governor of Hyogo prefecture, which includes Kobe.

After a crisis cabinet meeting on Monday, Prime Minister Taro Aso urged the public to stay calm, stressing that most people recover quickly from the virus.
"Caution is needed because there are cases of patients with chronic diseases such as diabetes whose condition is serious," Aso said.

"At the moment, however, many people are recovering smoothly after receiving appropriate treatment early," Aso said, adding that the government had no immediate plans to restrict people's movements and activities.
Japan's two main news agencies counted 135 cases of swine flu Monday but the Japanese Kyodo agency later said the figure had been revised up to 163 after 67 new infections were confirmed Monday in Osaka and Hyogo prefectures.
Masuzoe, in a Tokyo media briefing several hours earlier, said the central government had at that stage confirmed 125 cases -- excluding four cases where Japanese nationals caught the virus in North America.

In Kobe, many office workers, shop staff and public transport officials were wearing masks on the first working day since the outbreak hit the city. Public health announcements were broadcast via train station loudspeakers.
Hotel manager Kotaro Takeda said: "We have received more than 120 cancellations, mostly because of the influenza. We are concerned that groundless rumours may affect our business further."

At Kobe High School, where the first domestic infection was confirmed, all classrooms and schoolyards were empty, while teachers in staff rooms phoned students one by one to check whether they have flu symptoms.
The principal, Yukihiro Amano, told reporters: "We have done whatever we could, but the situation is beyond our expectations."

A housewife living nearby, 31-year-old Yoshiko Fujii, told AFP: "Now that the influenza is spreading so fast, I guess I have to be ready to be infected."
But other residents cautioned against over-reacting to the virus.
"I see the outbreak positively as the virus appears less dangerous than originally thought," said Kozue Yamagata, a magazine editor. "It's a good opportunity to learn how to act before we may face a more serious one."

The government will begin running television messages as early as Tuesday in which Aso will call on people to remain calm amid the outbreak, the Cabinet Office said, according to Kyodo.
No cases have yet been reported in greater Tokyo, which with almost 36 million people is the world's most populous urban area.

Japan's first confirmed cases of swine flu were four people who tested positive after they flew in from North America earlier this month. They were immediately quarantined along with about 50 fellow passengers.