A former sports doctor who admitted molesting some of the United States' top gymnasts for years has been sentenced to 40 to 175 years in prison by a judge who proudly told him: "I just signed your death warrant."

Three key leaders at USA Gymnastics have resigned as more women and girls told a judge they were sexually assaulted by a sports doctor who spent years with Olympic gymnasts and other female athletes.

A judge is holding an extraordinary four-day sentencing hearing for a disgraced American sport doctor who sexually assaulted young elite gymnasts.

USA Gymnastics has said it is "very sorry" that young athletes were abused by its former national team doctor Larry Nassar on the day he pleaded guilty to seven counts of sexual assault.