A reserve member of the U.S. women's gymnastics team has tested positive for the coronavirus during her pre-Tokyo Olympic training camp, officials said Monday.

The name of the athlete, who is in her teens, has been withheld by the city of Inzai, Chiba Prefecture, where she was training ahead of the opening of the Tokyo Olympics on Friday.

While several athletes from overseas have tested positive for the virus since arriving in Japan, it is the first known COVID case among Olympians from the United States.

Another gymnast is isolating at a hotel room after health authorities deemed that the athlete had been in close contact with her, the city said.

In a statement, the U.S. Olympic and Paralympic Committee said the reserve athlete has been transferred to a hotel to quarantine in line with local rules and protocols, and did not provide further information due to privacy reasons.

"The health and safety of our athletes, coaches and staff is our top priority," it said.

The gymnast was suspected of being infected with the virus on Sunday and another test at a hospital early Monday confirmed the result, according to the city.

The team of 10 athletes have been screened for the virus daily since arriving in Japan on Thursday, and only went out for training using a designated bus, the city said.

The Japanese organizers and the International Olympic Committee have put in place what they call "strict" anti-COVID-19 measures to prevent the spread of the virus during the games, due to open following a one-year postponement because of the coronavirus pandemic.

Nonetheless, a number of positive cases have been reported in and outside the athletes' village, while many people have been isolated after they were deemed as close contacts, reigniting worries about whether the games can be held safely, as promised by the organizers, during a global health crisis.

On Monday, the organizing committee said it has identified 21 people as close contacts of three South African men's soccer team members who tested positive at the athletes' village.

The news of the gymnast's infection comes after American tennis star Coco Gauff said on Twitter that she has pulled out of the Tokyo Games after testing positive for the virus.

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