"The EU joins growing international demands, including by sport professionals, for assurances that she is free and not under threat," the EU's foreign service said in a statement.

"The EU requests the Chinese government to provide verifiable proof of Peng Shuai's safety, wellbeing and whereabouts," the statement said.

The bloc also urged that the Chinese authorities carry out a "full, fair and transparent investigation into her allegations of sexual assault".

The whereabouts of Peng, a former doubles world number one, became a matter of concern when she disappeared on Nov. 2 after posting a message on social media alleging that China's former Vice-Premier Zhang Gaoli had sexually assaulted her.

She re-appeared in mid-November in Beijing and held a video call with International Olympic Committee (IOC) president Thomas Bach. But the Women's Tennis Association (WTA) has said this did not address or alleviate concerns about her health and safety.

A phone number at the Chinese embassy in Brussels was not answered on Tuesday. Neither Zhang nor the Chinese government have commented on Peng's allegations, although Beijing has told foreign governments not to interfere in its affairs. The topic has been blocked from direct discussion on China's internet.