The White House and the United Nations have joined demands to establish the whereabouts and welfare of missing Chinese tennis player Peng Shuai.

Peng has not been seen since making allegations of sexual assault against a former vice-premier in China two weeks ago, creating concern for her safety and wellbeing.

White House press secretary Jen Psaki told reporters: "We are deeply concerned by reports that Peng Shuai appears to be missing after accusing a former PRC (Peoples Republic of China) senior official of sexual assaults.

"We join in the calls for PRC authorities to provide independent and verifiable proof of her whereabouts and that she is safe."

Liz Throssell, the spokesman for the UN Human Rights Office, echoed those concerns and called for an investigation "with full transparency" into Peng's allegations.

Throssell told reporters: "(Peng) hasn't been heard from publicly since she alleged on social media that she was sexually assaulted.

"What we would say is that it would be important to have proof of her whereabouts and wellbeing, and we would urge that there be an investigation with full transparency into her allegations of sexual assault."

World number one Novak Djokovic has said tennis must stand together and described the ongoing situation as horrific.

Asked for his thoughts following his victory over Cameron Norrie at the ATP Finals, Djokovic said: "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 the Chinese soil without resolving this situation, it would be a little bit strange."

Serena Williams joined the calls for an investigation on Thursday night, with the 23-time grand slam singles champion tweeting she was "devastated" to hear the news about Peng.

Serena Williams has expressed her concern

And on Friday, the Lawn Tennis Association offered its support to the women's sport's governing body, the WTA, which has been vociferous in its calls for the Chinese authorities to address the situation.

"This is a very concerning situation, and we have written to the WTA offering our assistance in their efforts to establish the safety and wellbeing of Peng Shuai," an LTA statement read.

"We are also keen to support any further measures that the tours can introduce to improve the safety of all players.

"The immediate priority is to establish that Peng Shuai is safe and well and, furthermore, that she is able to speak freely and not subject to any form of censorship."

The International Olympic Committee declined to comment on the matter, with Beijing set to host the Winter Olympics in February.

An IOC spokesperson said: "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."