The United States and United Nations demanded proof of Peng Shuai's whereabouts as a Chinese state-affiliated journalist shared unverified photos of the tennis star, who has not been seen publicly since accusing a former vice-premier of forcing her to have sex.

Images of a smiling Peng emerged on a Chinese state-affiliated Twitter account late Friday, but their authenticity could not be verified and the user did not respond to AFP's request for comment.

Earlier 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 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 -- the first time China's #MeToo movement has touched the highest echelons of the country's politics.

Peng, 35, alleged on the Chinese social media site Weibo earlier this month that Zhang, now in his 70s, had "forced" her into sex during an on-off relationship spanning several years.

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

"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.

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

Other tennis stars, sports bodies, governments and human rights defenders have added to the growing clamour for information.

The head of the WTA, the top world body for women's tennis, has said the organisation is willing to lose hundreds of millions of dollars worth of Chinese business in one of its biggest markets to ensure Peng's wellbeing.

China has repeatedly refused to comment on the case.

On Saturday, Hu Xijin, the outspoken editor of the Communist Party-owned Global Times, tweeted: "she will show up in public and participate in some activities soon."

- Unverified photos emerge -

The four undated photographs that emerged late Friday were posted by the Twitter account @shen_shiwei, labelled "Chinese state-affiliated media" by the social network.

The user said the pictures were shared on Peng's WeChat Moments, a function often restricted to friends, to wish her followers a "good weekend".

One image shows a smiling Peng holding a cat in her arms, with stuffed animals, a trophy and a Chinese flag visible in the background.

Another picture has an image of Winnie the Pooh in the background. The children's character is often censored online in China because critics say Chinese leader Xi Jinping resembles the cartoon.

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

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

But doubts were flagged about the awkward language and the cursor visible in the screenshot.

Twitter is a blocked social network in China, and only people with a VPN-type workaround can access it.

- Tennis community rallies -

On Saturday, Craig Tiley, CEO of Tennis Australia and tournament chief of the Australian Open, became the latest figure from the tennis world to weigh in.

"The primary thing for us is to make sure that she's safe," he said.

"We've utilised the channels that we have working behind the scenes to find out, to get more clarity on her safety."

US Open Tennis also posted a message of solidarity on its Twitter account, saying "We stand with the tennis community in the concern for Peng Shuai's health and safety".

The grand slam joined the chorus of social media posts asking #WhereIsPengShuai, with tennis players past and present using the hashtag to voice concern for her safety.

On Friday Novak Djokovic said he  "absolutely" backed the WTA's threat to pull business from China.

"If you have tournaments on Chinese soil without resolving this situation it would be a little bit strange," the 20-time Grand Slam champion said.

Serena Williams and Naomi Osaka have also voiced their concerns for one of China's greatest ever players, with the American calling for an investigation.

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.

Amnesty International said "The Chinese government has systematically silenced the country's #MeToo movement," in a statement published Thursday.

"It is deeply concerning that Peng Shuai appears to be missing after accusing a high-ranking former government official of sexual assault."