The former doubles world number one has not been seen or heard from publicly since she said on Chinese social media that former vice premier Zhang Gaoli coerced her into sex.

Peng, 35, alleged  that former vice premier Zhang had
Peng, 35, alleged that former vice premier Zhang had "forced" her into sex during a long-term on-off relationship. (Vincent Thian / AP)

The United Nations has asked for proof of the whereabouts of Chinese tennis star Peng Shuai, who has been missing ever since she made sexual assault allegations against a former vice premier.

The UN Human Rights Office called for a fully transparent investigation on Friday into the former Grand Slam doubles champion's claims against Zhang Gaoli.

"It would be important to have proof of her whereabouts and well-being and we would urge that there be an investigation with full transparency into her allegations of sexual assault," Liz Throssell, a spokeswoman for UN rights chief Michelle Bachelet's office, told reporters in Geneva.

Throssell also said that 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 would stress that it's important to know where she is and her state, know about her well-being,'' Throssell added while indicating it would be important that there's an investigation into her allegations.

China's Foreign Ministry spokesperson Zhao Lijian told reporters on Friday that the matter was “not a diplomatic question and I’m not aware of the situation.”

READ MORE: WTA threatens to pull out of China as concerns for Peng grow

The allegations

Peng, 35, a former Wimbledon and French Open doubles champion, alleged on the Chinese social media site Weibo earlier this month that former vice-premier Zhang had "forced" her into sex during a long-term on-off relationship.

The unverified claims were quickly scrubbed from the Twitter-like platform and she has not been seen since, drawing mounting concern over her well-being.

Throssell said it was often very difficult for victims of sexual assault to come forward with their allegations.

"Sexual assault is found in any and every society," she said.

"With regard to the case of Peng Shuai, we are calling for an investigation with full transparency into her allegation of sexual assault.

"That should be the case into all allegations of sexual assault. It is really important to ensure accountability and justice for the victims, people who have suffered terrible trauma.

READ MORE: Tennis world concerned over whereabouts of China's Peng Shuai

Source: AFP