Japanese authorities quarantined the 46-year-old man and 16-year-old boys, who had been on a high school trip to Canada, and 49 others who were aboard their flight as it arrived in Tokyo on Friday from the US city of Detroit.

Seven of the 49 were later taken to hospital as they had flu symptoms, a health ministry official said.

The seven -- all the students' schoolmates -- tested negative in preliminary examinations, "but they are now undergoing genetic tests for the new type influenza", a ministry spokesman said.

In a separate case, a woman in her 30s recently back from the United States was tested for swine flu in Kawasaki, near Tokyo, but was found not to have the disease, Japanese news agency Kyodo quoted city authorities as saying.

Prime Minister Taro Aso said the three confirmed cases were not considered a domestic outbreak of the A(H1N1) virus because they were intercepted at the airport.

"I would like people to act calmly but stay alert while paying attention to information given by the government and local authorities," he said.

Later in the day, Aso told reporters his government was ready to beef up epidemic prevention measures should a regional outbreak of the virus be confirmed in Asia.

The government is trying to contact for testing all 409 people, including 21 crew, who were on the Northwest Airlines flight, an official said.

"All the passengers who were aboard the plane could possibly have contracted the flu," Health Minister Yoichi Masuzoe said. "We want to track all of them down by this evening."

Seven of the 13 passengers from the flight who travelled on from Japan were found in the Chinese cities of Shanghai and Beijing and put into quarantine on Saturday, the Chinese state-run news agency Xinhua reported.

Another two were intercepted in Hong Kong and taken to hospital although neither showed any flu symptoms, a Hong Kong health official said.

One of the travellers was found at a Hong Kong hotel while another was stopped at the airport after landing from Taiwan, the health department spokesman said.

Japanese authorities said the affected teacher and pupils, from the western city of Osaka, and their 49 fellow passengers, whose nationalities were not immediately released, were taken to a hotel near the airport to be isolated for the next 10 days.

"There are limitations as we can have cases in which people show symptoms after leaving aircraft," Masuzoe said. "We have to carry out our crisis management, considering that a domestic infection is a matter of time."

The teacher and students had stayed in Oakville, west of Toronto, on a school trip from April 24 until last Thursday. The teacher remained sick while the two students were recovering, officials said.

Canada has the world's third highest number of swine flu infections, with 224 cases, and on Friday reported its first death.

On the same day, the United States overtook Mexico, the epicentre of the outbreak, to become the country with the most patients, 1,639 cases in 43 states, according to statistics released in the US.

On Friday, the Japanese government said a six-year-old boy living in Chicago had become the first Japanese national to have contracted the virus but had already recovered.

Japan has been on high alert this week as one of the year's busiest travel periods came to an end, the "Golden Week" of public holidays when tens of millions travel domestically and overseas.

The country's leading expert on infectious diseases Nobuhiko Okabe Friday urged more medical measures to prepare for a domestic outbreak.

Greater Tokyo, with almost 36 million people, is the world's most populous urban area, according to United Nations data, far ahead of New York-Newark, Mexico City, Mumbai and Sao Paulo.

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