Simone Biles watched as her friends and former Olympic teammates came forward to detail abuse at the hands of a now-imprisoned former USA Gymnastics team doctor.

Drawing in part from their strength, the four-time gold medallist acknowledged Monday she is among the athletes who were sexually abused by Larry Nassar.

Biles, who won five medals overall at the 2016 Olympics, released a statement via Twitter outlining that abuse. Nassar, who spent more than two decades as a physician at USA Gymnastics while also working at Michigan State University, has admitted sexually assaulting gymnasts, possessing child pornography and molesting girls who sought medical treatment. He was sentenced in December to 60 years in federal prison for possessing child pornography and is facing another 40 to 125 years in prison after pleading guilty to assaulting seven girls.

Biles, now 20, called Nassar's behaviour "completely unacceptable, disgusting, and abusive, especially from someone whom I was told to trust." She joins a list of high-profile gymnasts who have come out against Nassar, including six-time Olympic medallist Aly Raisman, 2012 all-around champion Gabby Douglas and two-time Olympic medallist McKayla Maroney.

Like her Olympic teammates, Biles detailed abuse by Nassar that he disguised as treatment.

"It is not normal to receive any type of treatment from a trusted team physician and refer to it horrifyingly as the 'special' treatment," Biles wrote.

Biles is in the beginning stages of a return to competition, a journey that includes visits to the national team's training centre at the Karolyi Ranch north of Houston, where she said the abuse occurred.

"It is impossibly difficult to relive these experiences and it breaks my heart even more to think that as I work towards my dream of competing in Tokyo 2020, I will have to continually return to the same training facility where I was abused," Biles wrote.

USA Gymnastics initially agreed to buy the Karolyi Ranch in the summer of August 2016, following the retirement of longtime national team co-ordinator Martha Karolyi, but then backed out of the deal. The national team continues to use the facility while options for a replacement are explored.

Biles says she initially wondered if she was to blame.

"For too long I've asked myself, 'Was I too naive? Was it may fault?"' Biles wrote. "I now know the answer to those questions. No. No. It was not my fault. No, I will not and should not carry the guilt that belongs to Larry Nassar, USAG, and others."

USA Gymnastics said in a statement it is "heartbroken, sorry and angry" that Biles and other athletes were harmed by Nassar.

"USA Gymnastics' support is unwavering for Simone and all athletes who courageously came forward to share their experiences," the organization said in a release. "We are our athletes' advocates. USA Gymnastics will continue to listen to our athletes and our members in our efforts of creating a culture of empowerment with a relentless focus on athlete safety every single day."

The organization has taken several steps in recent months. President and CEO Steve Penny resigned under pressure last March and was replaced by Kerry Perry, who took over on Dec. 1.

The organization hired Toby Stark, a child welfare advocate, as its director of SafeSport last summer. Part of Stark's mandate is educating members on rules, educational programs and reporting. The federation also adopted over 70 recommendations by Deborah Daniels, a former federal prosecutor who oversaw an extensive independent review.

That's not far enough for some. Raisman has urged the organization to remove chairman of the board Paul Parilla among others. Biles, like Raisman, wants USA Gymnastics to take a deeper look at the conditions that allowed Nassar's behaviour to run unchecked for so long.

"We need to know why this was able to take place for so long and to so many of us," Biles said. "We need to make sure something like this never happens again."