Japan said Sunday that nine more high school students had tested positive for swine flu, bringing the total number of domestic infections to 17 amid fears the virus has a foothold in the community.
The latest confirmed cases are all students in Osaka prefecture in western Japan, a health ministry official told AFP.
Eight students had previously tested positive in Kobe, in the adjoining prefecture of Hyogo.
"We've confirmed a total of nine new cases in Osaka prefecture, and are in the process of testing one more case," the official said, adding work was ongoing to gather information on the route of the infection through interviews with doctors and families of the patients.
The new cases come as the World Health Organisation (WHO) said Saturday it is closely monitoring swine flu in Japan after officials ordered schools closed and cancelled events in Kobe.
A local official in Osaka said: "So far we can't find clear records of contacts with students in Osaka and students in Kobe, and they have not travelled abroad recently."
In Osaka, about 100 more high school students who had displayed suspicious symptoms such as fever are to be tested, officials said.
Meanwhile seven more people are suspected to be infected with swine flu in Hyogo prefecture, officials said.
These people -- a teacher, a university student and five high school students -- had recently had contact with one of the
students in Kobe who tested positive, public broadcaster NHK reported.
Masato Tashiro, a Japanese virologist who is in Geneva as a member of the WHO's panel on the epidemic, told public broadcaster NHK: "I don't know specifics about the cases (in Japan) but judging from confirmed cases the infection is likely to be spreading to hundreds of people."
"There must be a number of people who slipped through border controls (at airports) as their symptoms were quite light, passing the virus to other people before they knew it," he said.
Prime Minister Taro Aso has called on the country to keep calm.
But former WHO senior official Shigeru Omi, now head of the government's special swine flu task force, warned that "we believe that the infection is beginning to spread in the region."
Japan on May 9 confirmed its first A(H1N1) cases contracted overseas -- a school teacher and three students who flew to Tokyo from Canada via Detroit -- but immediately quarantined them and other passengers.
All four have since recovered.