Bela Karolyi, the charismatic if polarising gymnastics coach who turned young women into champions and the United States into an international power, has died. He was 82.

USA Gymnastics said Karolyi died on Friday. No cause of death was given.

Karolyi and wife Martha trained multiple Olympic gold medallists and world champions in the US and Romania, including Nadia Comaneci and Mary Lou Retton.

"A big impact and influence on my life," Comaneci, who was just 14 when Karolyi coached her to gold for Romania at the 1976 Montreal Olympics, posted on Instagram.

The Karolyis defected to the United States in 1981 and over the next 30-plus years became a guiding force in American gymnastics, though not without controversy. Bela helped guide Retton - all of 16 - to the Olympic all-around title at the 1984 Games in Los Angeles and memorably helped an injured Kerri Strug off the floor at the 1996 Games in Atlanta after Strug's vault secured the team gold for the Americans.

Karolyi briefly became the national team coordinator for USA Gymnastics women's elite program in 1999 and incorporated a semi-centralised system that eventually turned the Americans into the sport's gold standard.

It did not come without a cost. He was pushed out after the 2000 Olympics after several athletes spoke out about his tactics.

It would not be the last time Karolyi was accused of grandstanding and pushing his athletes too far physically and mentally.

During the height of the Larry Nassar scandal in the late 2010s - when the disgraced former USA Gymnastics team doctor was effectively given a life sentence after pleading guilty to sexually assaulting gymnasts and other athletes with his hands under the guise of medical treatment - over a dozen former gymnasts came forward saying the Karolyis were part of a system that created an oppressive culture that allowed Nassar's behaviour to run unchecked for years.

While the Karolyis denied responsibility - telling CNN in 2018 they were unaware of Nassar's behaviour - the revelations led to them receding from the spotlight.

Still, some of Karolyi's most famous students were always among his staunchest defenders.

When Strug got married, she and Karolyi took a photo recreating their famous scene from the 1996 Olympics, when he carried her onto the medals podium after she vaulted on a badly sprained ankle.

He could be a harsh taskmaster, calling his gymnasts names, taunting them for their weight and pushing them to their limits.

Even those warm embraces weren't always quite what they seemed.

"A lot of those big bear hugs came with the whisper of 'Not so good,' in our ears," Retton wrote.

Yet Retton and Comaneci remained close with Karolyi, making appearances with him at gymnastics events or sitting with him at competitions.

Dominique Moceanu, part of the "Magnificent 7" team that won gold in Atlanta, talked extensively about her corrosive relationship with the Karolyis following her retirement. In her 2012 memoir, Moceanu wrote Bela Karolyi verbally abused her in front of her teammates on multiple occasions.

"His harsh words and critical demeanor often weighed heavily on me," Moceanu posted on X Saturday. "While our relationship was fraught with difficulty, some of these moments of hardship helped me forge and define my own path."