The United Nations said on Friday it wants proof of the whereabouts of Chinese tennis star Peng Shuai, missing ever since she made sexual assault allegations against a former vice-premier.

The UN Human Rights Office called for a fully-transparent investigation 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.

“According to available information, Peng, a former world doubles number one, hasn’t been heard from publically 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.

“We think it would be important that there’s an investigation into her 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 claims were quickly scrubbed from the Twitter-like platform and she has not been seen since, drawing mounting concern over her wellbeing.

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.

“Very often it is such a challenge for them to come forward and to make these allegations public.”