TOKYO - Kara Eaker, an Olympic reserve on the US women's gymnastics team, has tested positive for COVID-19 at a training camp near Tokyo.

Eaker, 18, is the first American Olympian to test positive.

Eaker's father Mark told media on Monday that his daughter tested positive at the team's training camp in Inzai, 30 kilometres east of Tokyo, and added she was asymptomatic and had been fully vaccinated for COVID-19.

"We confirmed today that a teen athlete of the team has tested positive," said Kimiya Kosaku, an Inzai city official, adding that she had arrived in Japan on July 15.

"Another athlete has been categorised as a close contact and is also staying alone inside her room," Kosaku said.

Eaker's personal coach Al Fong also confirmed the positive test on Monday.

The result came after Eaker received what was described as a "false positive" over the weekend. Eaker took a subsequent test that was negative before testing positive again multiple times.

Four alternates, including Eaker, traveled to Japan with the six-woman US Olympic gymnastics team of world and Olympic champion Simone Biles, Jordan Chiles, Grace McCallum, Sunisa Lee, MyKayla Skinner and Jade Carey.

The alternates have been rooming with other alternates since arriving in Japan, with the competitive team rooming with fellow competitors.