BEIJING -- China's Foreign Ministry stuck to its line Friday that it wasn't aware of the controversy surrounding Peng Shuai, the tennis professional who disappeared after accusing a former top official of sexually assaulting her.

Ministry spokesperson Zhao Lijian told reporters the matter was "not a diplomatic question and I'm not aware of the situation."

The ministry has consistently disavowed knowledge of the issue since Peng made her accusation more than two weeks ago.

The 35-year-old former top-ranked player in women's doubles won titles at Wimbledon in 2013 and the French Open in 2014. Peng also participated in three Olympics, making her disappearance all the more prominent with Beijing set to host the Winter Games starting Feb. 4.

White House press secretary Jen Psaki said Friday that Chinese authorities should "provide independent and verifiable proof of her whereabouts and that she is safe."

U.S. Deputy Secretary of State Wendy Sherman tweeted: "We are deeply concerned by reports that tennis player Peng Shuai appears to be missing, and we join the calls for the PRC to provide independent, verifiable proof of her whereabouts. Women everywhere deserve to have reports of sexual assault taken seriously and investigated."

Liz Throssell, a spokeswoman for the U.N. human rights office in Geneva, said Friday it was calling for "an investigation with full transparency into her allegation of sexual assault."

"And I think we would say that that should be the case into all allegations of sexual assault. It is really important to ensure accountability, to ensure justice for the victims," she said.

The International Olympic Committee declined to comment Friday, saying in an emailed statement: "Experience shows that quiet diplomacy offers the best opportunity to find a solution for questions of such nature. This explains why the IOC will not comment any further at this stage."

Top-ranked men's player Novak Djokovic, one of several tennis stars to speak up about Peng's situation, said he would support a decision to cancel tournaments in China unless the issue is resolved.

"This is horrifying. I mean, a person is missing," Djokovic said at the ATP Finals in Turin, Italy. "The whole community, tennis community, needs to back her up and her family, make sure that she's safe and sound because if you would have tournaments on Chinese soil without resolving this situation, it would be a little bit strange."

Peng wrote in a lengthy social media post on Nov. 2 that she was forced to have sex three years ago with Zhang Gaoli in his home despite repeated refusals. Zhang, 75, is a former vice premier who was a member of the ruling Communist Party's all-powerful Politburo Standing Committee.

The post was quickly deleted from Peng's verified account on Weibo, a leading Chinese social media platform, but screenshots of the explosive accusation were shared on the internet.

Steve Simon, the chairman and CEO of the Women's Tennis Association, questioned the authenticity of what a Chinese state media outlet said this week was an email intended for him in which Peng said she was safe and that the assault allegation was untrue. It was tweeted by CGTN, the international arm of Chinese state broadcaster CCTV.

The State Council Information Office, which represents the Chinese government, did not respond to emailed questions about Peng's current situation and Simon's doubts about the email.

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Associated Press writer Jamey Keaten in Geneva contributed to this report.