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

Peng (35) 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."

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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."

The Women's Tennis Association has threatened to pull out of the lucrative Chinese part of its tour unless it is proved that Peng is safe.

WTA chairman Steve Simon told the BBC: "We cannot stand by compromises. This is a right and wrong issue."

Men's world number one Novak Djokovic backed that stance following his win over Cameron Norrie at the ATP Finals in Turin.

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He said: "It's important because this is horrifying. I mean, a person is missing. "China is a huge country. It's a very important part of the world especially for the WTA. They have many tournaments there. I mean, this is necessary for us to take whatever action.

"I heard just now that the WTA is willing to pull out from China with all the tournaments unless this is resolved. I support it 100 per cent."

The Lawn Tennis Association earlier offered its support to the WTA's strong 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 tiptoed around 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."

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.