(AFP) - The United States and UN on Friday demanded proof of Peng Shuai's whereabouts and well-being amid rising concern for the tennis star, missing since alleging she was sexually exploited by a former vice-premier of China.

White House press secretary Jen Psaki said President Joe Biden's administration wanted China to "provide independent, verifiable proof" of Peng's whereabouts and expressed "deep concern" about the former world top-ranked doubles player.

The United Nations insisted on a fully transparent investigation into the claims made by Peng against Communist Party grandee Zhang Gaoli.

Tennis stars, sports bodies, governments and human rights defenders also spoke up for Peng, 35, and demanded information.

The head of the Women's Tennis Association (WTA) said he was prepared to cut lucrative business ties with China if Peng remains unaccounted for and her sexual assault allegations are not probed.

Novak Djokovic said he backed the WTA's threat.

"I hope that she's found and that she's healthy and that everything is fine, at least the worst is avoided. I support the statement of the WTA as an organisation and also their president absolutely," said Djokovic, a 20-time Grand Slam champion.

"The whole tennis community needs to back her up and her family and make sure that she is safe and sound. Because if you have tournaments on Chinese soil without resolving this situation it would be a little bit strange, so I understand why the WTA has taken a stance like that."

Serena Williams and Naomi Osaka have also voiced their concerns for one of China's greatest ever players.

"It would be important to have proof of her whereabouts and well-being," Liz Throssell, a spokeswoman for the UN Human Rights Office, told reporters in Geneva.

"According to available information, Peng, a former world doubles number one, hasn't been heard from publicly since she alleged on social media that she had been sexually assaulted.

"We are calling for an investigation with full transparency into her allegation of sexual assault."

- 'Women need to be respected' -

Peng alleged on the Chinese social media site Weibo earlier this month that Zhang, now in his 70s, had "forced" her into sex during a long-term on-off relationship.

The claims were quickly scrubbed from the Twitter-like platform and she has not been seen since.

The WTA, the top world body for women's tennis, has called for proof that Peng is safe.

Its boss Steve Simon said he is willing to lose hundreds of millions of dollars worth of Chinese business in one of the WTA's biggest markets to ensure Peng's safety.

"We're definitely willing to pull our business and deal with all the complications that come with it," Simon told CNN.

"Women need to be respected and not censored," he added.

Tennis legend Serena Williams also demanded an investigation.

"I am devastated and shocked to hear about the news of my peer, Peng Shuai," the former singles world number one wrote on Twitter.

"This must be investigated and we must not stay silent."

Peng represented China in the Beijing, London and Rio de Janeiro Olympics and won gold for China at the 2010 Asian Games.

She is a former Wimbledon and French Open doubles champion.

France said the international community and sports bodies were worried for Peng's situation.

"We are concerned about the lack of information," the French foreign ministry said.

"We call on the Chinese authorities to implement their commitments in the fight against violence against women."

The Lawn Tennis Association, the sport's governing body in Britain, offered its assistance to the WTA in its efforts to establish Peng's safety.

- Email doubts -

Peng's claims brought the #MeToo movement into the highest echelons of China's ruling Communist Party for the first time.

"The Chinese government has systematically silenced the country's #MeToo movement," said Amnesty International's China researcher Doriane Lau.

"Given that it also has a zero-tolerance approach to criticism, it is deeply concerning that Peng Shuai appears to be missing," she said.

China has repeatedly refused to comment on her fate or the case.

But Hu Xijin, the outspoken editor of the Communist Party-owned Global Times, tweeted on Friday that he didn't believe that "Peng Shuai has received retaliation and repression speculated by foreign media for the thing people talked about".

Earlier this week, state-run CGTN published a screenshot on Twitter of what it said was an email written by Peng to Simon and other WTA officials.

In the email, Peng claims that her earlier accusations are "not true" and says she is "resting at home and everything is fine".

But doubts were quickly flagged about the awkward language used in the purported email and the cursor visible in the screenshot.

© Agence France-Presse