Japan said Sunday that 20 more people had tested positive for swine flu, bringing the total number of cases to 32 amid fears the virus has a foothold in the west of the country.
The latest group of flu sufferers included 18 high school students in Osaka prefecture and the city of Kobe in neighbouring Hyogo prefecture, where eight students were already ill from the virus, a health ministry official told AFP.
A college student in his 20s and a teacher in his 40s, both in Kobe, also tested positive for A(H1N1) influenza, he said.
"Now the number of cases of domestic infection has risen to 28," the official said.
Four other Japanese, a school teacher and three students who flew to Tokyo from Canada via Detroit, contracted the virus overseas earlier this month and have since recovered.
"We quickly need to collect information on the current infection," said Chief Cabinet Secretary Takeo Kawamura, the top government spokesman.
"We are studying how to prevent the spread of infection," Kawamura told reporters, adding that the cabinet would hold an emergency meeting on Monday to discuss the issue.
Prime Minister Taro Aso has called on the country to remain calm.
But Shigeru Omi, a former senior official at the World Health Organization who is now head of the government's special swine flu task force, warned: "We believe that the infection is beginning to spread in the region."
The WHO said Saturday it was closely monitoring the swine flu situation in Japan after officials shut down schools and cancelled public events in Kobe, where people with flu symptoms were seeking treatment at local hospitals.
"I had never dreamed that the new type flu outbreak would happen in my city," said Seiji Koga, a 62-year-old construction company worker. "Since we can't move away, we have to spend restless days for now."
About 100 more high school students in Osaka and seven people in Hyogo prefecture who had displayed suspicious symptoms were to be tested, officials 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," said one local official in Osaka.
The seven people in Hyogo, 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 serves on an WHO 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.