The family of a missing employee of Britain's consulate in Hong Kong said Wednesday that authorities told them he was detained while returning from a business trip but do not know why, where or how long he will be held.
The consulate has refused to name the man or give details about the incident and a Chinese official said he was "not aware" of the situation.
But the family wrote on Facebook that his name is Simon Cheng and that he had traveled to Shenzhen in Guangdong province, an hour outside Hong Kong, for a one-day business meeting on August 8.
That night, Cheng returned via high-speed train and sent messages to his girlfriend as he was about to go through customs.
"We lost contact with him since then," according to the Facebook post.
It said Hong Kong's Immigration Department, after consulting with the city's trade office in Guangdong, "told us verbally that Simon has been administratively detained, but said that they could obtain no information on why, where and for how long he was to be detained."
The family said it has received no notice of administrative detention. Lawyers have confirmed that Cheng's case is being handled by the Shenzhen Municipal Public Security Bureau, but failed to ascertain where he is now, the family said.
"We feel very helpless, and are worried sick about Simon. We hope that Simon can return to Hong Kong as soon as possible," the family post said.
Hong Kong police confirmed they had opened a missing persons case on August 9.
Chinese foreign ministry spokesman Geng Shuang said: "I am not aware of the relevant situation."
- Weeks of rallies -
The incident comes as the financial hub faces its worst political crisis in decades.
Pro-democracy protesters have staged weeks of rallies that have often descended into violent clashes with police.
Beijing has taken an increasingly hard line against the protests, which it sees as a direct challenge to its rule.
It has also repeatedly warned Britain -- the former colonial ruler of Hong Kong -- against "interference" in the protests, and relations between the two countries have been increasingly strained over the issue.
The demonstrations were triggered by a controversial extradition law but have broadened into a call for wider democratic reforms.
China had promised to respect the freedoms in the semi-autonomous territory after its handover from Britain in 1997.
Hong Kong enjoys liberties unseen on the mainland, including freedom of speech, unfettered access to the internet and an independent judiciary.
But the ongoing protests have raised fears of a Chinese crackdown.
Known for its high-tech market, the metropolis of Shenzhen sits behind China's "Great Firewall", which restricts access to news and information.
With Beijing attempting to shape the narrative of the unrest in Hong Kong, Chinese authorities have increased their inspections at the border, including checking the phones and devices of some passengers for photos of the protests.