TOKYO: Japan confirmed dozens more swine flu cases overnight bringing its tally to 173, health officials said Tuesday, as the government considered scaling down quarantine checks at airports while focusing on its domestic outbreak.

The outbreak in Japan has surged from just four cases over the weekend after infections were confirmed in the port city of Kobe and nearby Osaka, which is Japan's second-biggest urban area. The 38 added to the list overnight included high school students and small children.

The new wave of infections did not have a clear connection to foreign travel, as the initial one did, and involved primarily teenagers. None of the patients were in serious condition.

The number could climb further because medical institutions around the country are now testing viral samples of suspected patients with no foreign travel records, who were previously excluded from testing.

Health and Welfare Minister Yoichi Masuzoe indicated that airport quarantine efforts would be scaled down and urged flexibility in anti-flu measures since the swine flu is now believed to be no more infectious than seasonal influenza. Health officials want to make sure that hospital beds are not clogged up with mild patients and that business and community activities are not paralyzed due to overreaction.

Chief Cabinet Secretary Takeo Kawamura told a news conference: "We need to prevent the infections from spreading further, we must also maintain public activities." However, the government said it would do its best to quell the spread of the disease, and keep schools shut at least through the end of the week. Companies discouraged unnecessary business trips, and schools have canceled excursions. Visitors to parliament and other government offices are requested to wear surgical masks and rub their hands with disinfectant gel before entry.

Japan had worked hard to keep the new flu strain from spreading within its island borders, but on Saturday, officials reported its first case of an infection not being brought in from abroad. It remained unclear where the bulk of the patients came into contact with the virus.

Reports said dozens of the students had played in school volleyball games, but officials could not confirm if that had any relation to their infections.

Experts also suspect that a domestic outbreak may have been triggered silently weeks before Japan caught its first cases at its main airport in early May.

Japan now ranks the fourth-most infected country in the world, after Mexico, the United States and Canada, according to Japanese media reports.

Officials admitted the new figures were disturbing.

Hyogo Gov. Toshizo Ido said health officials are having trouble tracing the route of infections that started from Kobe, which is part of Hyogo in Japan's southwest. Osaka Gov. Toru Hashimoto said he issued an epidemic warning in the prefecture.

According to local officials, 1,466 schools - all elementary, junior and senior high schools in Hyogo - were closed Monday through at least Friday. Several hundred more were closed in Osaka.

Private companies were also taking measures to stem the spread.

Companies including Shiseido, Daihatsu Motor Co. and Mitsubishi Heavy Industries Ltd. have reportedly ordered employees to limit nonessential domestic business trips, particularly to infected regions.

Health officials have warned that the new H1N1 swine flu virus could eventually infect millions of people. The World Health Organization has confirmed at least 8,829 human cases of swine flu in nearly 40 countries, including 76 known deaths.