Washington - Former USA Gymnastics doctor Larry Nassar - accused of sexually assaulting more than 100 young girls in his care, including members of the gold-medal winning US Olympic team - was sentenced to 60 years in prison on Thursday for child pornography.

The 54-year-old Nassar, who is facing sentencing next month on separate sexual assault charges, pleaded guilty in July to three counts of possessing child pornography.

The doctor, who worked at Michigan State University in addition to USA Gymnastics, was found to be in possession of more than 37,000 graphic videos and images of child pornography on his computer hard drives.

He was given the maximum 20 years in prison on each count by US District Judge Janet Neff at a federal court hearing in Grand Rapids, Michigan.

"He has demonstrated that he should never again have access to children," Neff was quoted by the local Lansing State Journal newspaper as saying.

"You have to wonder whether he felt he was omnipotent, whether he felt he was getting away with something so cleverly," she added.

Nassar said in court he was ashamed of his actions and had lost everything.

"You go back and you wonder how I got down this path to begin with," the Journal quoted him as telling the judge. 

"I'm going to take every day of your sentence to try to better myself."

Nassar has been accused of molesting more than 100 female athletes during the three decades he worked with USA Gymnastics and at Michigan State.

He has pleaded guilty to 10 counts of sexual assault in two other cases in Michigan, admitting that he abused young athletes under the guise of offering medical treatment.

He could face life in prison when he is sentenced on those charges next month. 

His case was part of a wide-ranging scandal which forced the resignation of USA Gymnastics chief Steve Penny in March.

Penny was accused by victims of failing to quickly notify authorities about abuse allegations.

Olympic gold medalists McKayla Maroney, Aly Raisman and Gabby Douglas are among the members of the USA Gymnastics team who have said they were sexually assaulted by Nassar.

Maroney, in a victim impact statement submitted to Neff, called Nassar a "monster" and urged her to hand down the maximum sentence.

"Dr. Nassar was not a doctor, he in fact is, was, and forever shall be, a child molester, and a monster of a human being," Maroney wrote. 

"He abused my trust, abused my body and left scars on my psyche that may never heal," she said.

"Larry Nassar deserves to spend the rest of his life in prison," she added.

Maroney won team gold and an individual vault silver medal at the 2012 London Olympics as a member of a US women's gymnastics team dubbed the "Fierce Five."

A three-time world champion, she went public with her allegations against Nassar amid the "#MeToo" movement.

She said Nassar began abusing her when she was just 13 and the molestation continued throughout her gymnastics career.

Her teammate, Raisman, released a statement on the sports website The Players' Tribune and expressed hope that Nassar's conviction would lead to changes.

"We must look at the organisations that protected Nassar for years and years: USA Gymnastics, the US Olympic committee and Michigan State University," Raisman said. 

"Until we understand the flaws in their systems, we can't be sure something like this won't happen again." 

USA Gymnastics adopted a new "safe sport policy" in response to the Nassar scandal that requires "mandatory reporting" of suspicions of sexual abuse.

A civil lawsuit has been filed on behalf of about 100 of Nassar's victims.

Their attorney, John Manly, estimated that the total number could be as high as 160.