The Chinese government has confirmed that it is holding a Taiwanese pro-democracy activist and is investigating him on suspicion of "pursuing activities harmful to national security".
Lee Ming-che, 42, cleared immigration in the semi-autonomous Chinese territory of Macau on March 19 and never showed up for a planned meeting later that day with a friend in the mainland Chinese city of Zhuhai.
China's Taiwan Affairs Office said Mr Lee was in good health, but gave no information about where he was being held or other terms of his detention.
"Regarding Lee Ming-che's case, because he is suspected of pursing activities harmful to national security, the investigation into him is being handled in line with legal procedures," spokesman Ma Xiaoguang told reporters at a news briefing.
Amnesty International said Mr Lee's detention raises fears China is broadening its crackdown on legitimate activism, and urged the authorities to provide further details on his detention.
Mr Lee's "detention on vague national security grounds will alarm all those that work with NGOs in China. If his detention is solely connected to his legitimate activism he must be immediately and unconditionally released," Nicholas Bequelin, the group's east Asia director, said by email.
Responding to Mr Ma's comments, Taiwan's Cabinet-level Mainland Affairs Council said repeated requests have been made to China through both official and private channels for information about Mr Lee, but none has been forthcoming.
It said he suffered from high blood pressure and other health problems, and asked that China "please provide the appropriate medical care and ensure his physical health".
A colleague of Mr Lee's said he may have attracted the attention of China's security services after he used the social media platform WeChat to discuss China-Taiwan relations.
Cheng Hsiu-chuan, president of Taipei's Wenshan Community College where Mr Lee has worked for the past year as a programme director, said Mr Lee used WeChat to "teach" an unknown number of people about China-Taiwan relations under the government of Taiwan President Tsai Ing-wen.
"For China, the material he was teaching would be seen as sensitive," Mr Cheng said.
WeChat has hundreds of millions of active users and is hugely popular in China, where other social media tools such as Twitter are blocked by the authorities.
Mr Lee had travelled annually to China for the past decade to see friends, Mr Cheng said.
He would discuss human rights in private, but had never held any public events there, Mr Cheng added.
However, in mid-2016 Chinese authorities shut down Mr Lee's WeChat account and confiscated a box of books published in Taiwan on political and cultural issues, Cheng said.
On his most recent trip, Mr Lee planned to see friends and obtain Chinese medicine for his mother-in-law in Taiwan, his wife, Lee Ching-yu, said.
He was expected to stay in the southern Chinese city of Guangzhou until March 26, she said.
"I want the government of China to act like a civilized country and tell me what they're doing with my husband on what legal grounds and, like a civilized country, what they plan to do with him," Ms Lee Ching-yu said.
China claims sovereignty over Taiwan, a free-wheeling democracy with personal and political freedoms largely unknown on the authoritarian, Communist-ruled mainland.
China insists that the two sides must eventually unify and has raised pressure on Taiwan since the election last year of President Tsai, whose Democratic Progressive Party advocates for Taiwan's formal independence.
China and Taiwan split amid civil war in 1949.