Officials said that a four-month-old US baby girl was undergoing tests for swine flu in Japan, the country's latest suspected case of the virus.
The baby had tested positive for type-A flu strain in a preliminary examination shortly after she arrived at Tokyo's US Yokota Air Base from the United States with her family on Friday, the foreign ministry said.
Samples from the baby were sent to the National Institute of Infectious Diseases in Tokyo, and "they are now under examination to judge if they are the new flu virus," said foreign ministry official Toshiyuki Maeda.
It was not known when the results would be ready.
The baby, being treated in an isolated bedroom at the base, still has a temperature but other passengers, including her mother, had tested negative, officials and news reports said.
The plane, chartered by the US military, was en route from Seattle-Tacoma International Airport, with 260 to 270 military personnel and others aboard, Kyodo News reported.
The US side would also examine samples and it was expected to take five to eight days before the results were available, Kyodo said, quoting US military officials.
No confirmed cases of the new A(H1N1) flu virus have yet been reported in Japan.
On Friday, Japan reacted with relief to news that the country's first suspected case of swine flu, a 17-year-old high school student who recently visited Canada, had tested negative.
The alerts have come as Japan starts one of the busiest travel periods of the year, the "Golden Week" of public holidays when tens of millions of people crowd trains and airports to visit family and friends.