The authorities in Michigan on Wednesday charged former US national gymnastics team doctor Lawrence Nassar with 22 criminal counts of sexual assault, including of girls under the age of 13.

Nassar, 53, was arrested last November on charges of sexually assaulting a child in Michigan, where he is being held without bail. That case was not connected to athletics, officials said.

In December, he was indicted on federal child pornography possession charges.

Dozens of women are now suing USA Gymnastics over Nassar's alleged actions.

Michigan Attorney General Bill Schuette on Wednesday charged Nassar with multiple sexual assault charges in two separate counties -- including five counts involving victims under 13 years old. Each charge carries a sentence of up to life in prison.

Nassar is scheduled to be arraigned Thursday. 

His attorney declined AFP's request for comment, but the doctor has previously denied the allegations of sexual abuse, saying his treatments were legitimate medical practices.

"Dr Nassar preyed on these young girls, he used his status and authority to engage in horrid sexual assaults under the guise of medical procedures."

Nassar had served on the US gymnastics team as recently as 2015. Michigan State University, where he had his practice, fired him in September after multiple accusations surfaced. His medical license was suspended as of January 2017.

-- Investigation ongoing --

The investigation into Nassar is ongoing and the authorities are encouraging others who think they may be victims to come forward.

"The allegations of sexual assault against Dr Nassar continue to increase nearly every day," Michigan State University Police Chief Jim Dunlap said in a statement. 

Over the weekend, three former US national team gymnasts detailed Nassar's alleged sexual abuse in a report on CBS television's news program "60 Minutes."

Jamie Dantzscher, a member of the 2000 Sydney Olympic US bronze medal squad, was joined in coming forward publically by Jessica Howard, the US national rhythmic gymnastics champion from 1999 through 2001, and Jeanette Antolin, a US team member from 1995 to 2000. 

"He would put his fingers inside of me and move my leg around," Dantzscher, who had gone to Nassar for treatment of back pain, told CBS. 

"It happened all the way to the Olympics in Sydney, 'til I was 18," she said. 

John Manly -- a California attorney representing more than 40 women in a lawsuit against USA Gymnastics for failing to protect his clients -- says Nassar may have abused hundreds of girls over more than two decades, many at the ranch training headquarters of famed US coaches Bela and Marta Karolyi.

The Karolyis, in a statement to "60 Minutes" from attorney James Christian, denied creating a "toxic" environment that allowed the abuse to take place. 

"The Karolyis did not have any knowledge of any complaint from anyone concerning any athlete's alleged mistreatment by Dr Nassar until they learned of his dismissal from USA Gymnastics," the statement said.

Responding to the "60 Minutes" report, US Senator Dianne Feinstein on Wednesday called for tougher standards that would require amateur sports governing bodies to notify law enforcement of potential abuse.