Japan on Saturday confirmed its first domestic infection of swine flu after a male high school student who has never been abroad tested positive for the virus, the government said.
"Today, the first patient infected with the new-type influenza in the country was confirmed," Health Minister Yoichi Masuzoe told a news conference.
Two of the student's schoolmates, one boy and one girl, had also tested positive for swine flu in an initial genetic examination, which was being followed up by further tests, government officials said.
"The (three) people have no record of travelling abroad," Masuzoe said, adding that the students had been isolated and were being treated in a hospital in the western city of Kobe.
Seventeen other students from the school were also being testing after they complained about feeling unwell, local officials said, without suggesting any of them were being considered suspected cases at this stage.
Prime Minister Taro Aso called on Japan's people to stay calm and vowed to take necessary measures to prevent a wider outbreak.
"Please act calmly while being on alert," Aso said in a statement.
"The government will carry out thorough inspections on the patients and on the people close to them. We will take action to stop the infection from spreading."
The student from Kobe, who had suffered flu symptoms for several days since Monday, tested positive for the virus in the final test, Masuzoe said.
"The infected student still has a temperature, while one of the two suspected patients is recovering," a health ministry official said.
Authorities in Kobe decided to temporarily close at least 75 schools and kindergartens and cancel festivals and other public events in some districts of the city, where fear of an outbreak was growing rapidly.
"It's totally beyond our imagination," Seiichi Sakurai, of the city's health and welfare bureau, told reporters. "We couldn't have predicted it.
"The virus entered the country undetected. I'm afraid the infection may have already spread further."
Reiko Hamuro, a 42-year-old transport company employee from Kobe, told AFP: "All of a sudden, people started wearing masks today. It's scary because the cases came without any warning signs. I will try to avoid crowds."
Kobe, one of the nation's largest port cities with a population of 1.5 million, is still remembered for the 1995 Kobe earthquake, which killed some 6,500 people in the region.
Saturday's confirmed case is the first infection known to have taken place within the country.
"We are entering a new phase," Chief Cabinet Secretary Takeo Kawamura told an emergency meeting at the prime minister's office in Tokyo. "We will do our best to protect the safety and security of the people."
Earlier this month, Japan confirmed its first A(H1N1) cases -- a school teacher and three students who arrived at Tokyo's Narita International Airport from North America and were immediately quarantined.
One of those four patients was released from hospital Saturday afternoon, while the remaining three were due to discharged after another round of flu tests as early as Sunday, local media said.
Authorities also isolated in a hotel 48 people who had been on the same flight from Detroit as the infected passengers. All of them had been released by Saturday, officials said.