TOKYO - A Japanese woman who developed flu symptoms after returning from the United States has tested negative for swine flu, meaning Japan has yet to report its first case of the virus, officials said Tuesday.

The woman, in her 40s, had tested positive for type-A flu in a preliminary examination shortly after she arrived at Tokyo's Narita Airport from Los Angeles Monday afternoon.

However, further tests for the new (A)H1N1 influenza virus came back negative, a health ministry official said. Japan has yet to confirm any cases of swine flu, although several patients have tested positive for what turned out to be a different strain of the influenza A virus.

Another Japanese woman in her 40s who developed flu symptoms after returning from the United States tested negative for swine flu, local government officials said Monday.

An American baby isolated after arriving in Tokyo from the United States also tested negative.

Japan is on alert for the virus during 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.

The country said Monday it would triple the number of quarantine officers at Narita airport to try to prevent a swine flu outbreak.