A Taiwanese NGO worker who promoted democracy in China has gone missing after entering the mainland earlier this month, authorities said.
Taiwan said Friday that Chinese authorities had not responded to their enquiries on Lee Ming-cheh's whereabouts, as his wife pleaded for help to locate him.
Ties between China and Taiwan have worsened since President Tsai Ing-wen took office in May and Beijing has cut off all official communication with Taipei.
Taiwan has been self-ruling since 1949 following a civil war on the mainland, but it has never formally declared independence and Beijing still claims it as part of its territory.
Chinese authorities deeply mistrust Tsai's Democratic Progressive Party (DPP), which is traditionally pro-independence.
Lee, 42, works for a community college in Taipei and "lost contact" on March 19 after he entered the southeastern Chinese city of Zhuhai from Macau, said the Mainland Affairs Council (MAC), Taiwan's top China policy-making body.
A former DPP employee, Lee had been sharing "Taiwan's democratic experiences" with his Chinese friends online for many years and often mailed books to them, according to the Taiwan Association for Human Rights.
"It's unreasonable that a Taiwanese citizen has been unaccounted for in China for more than five days for no reason," his wife Lee Ching-yu said in a statement.
"If Lee Ming-cheh has been arrested, please tell me on what charges ... whether he is alive or dead, and where he is," she said, adding that Lee has been following human rights issues in China.
Lee has long supported civil society organisations and activists in China, according to Amnesty International, which added that he went there this time to arrange for his mother-in-law's medical treatment.
His disappearance "raises serious questions about the safety of people working with civil society in China", said Nicholas Bequelin, Amnesty International's East Asia Director.
Taiwan authorities say Zhuhai police have informed Lee's family that there is no record of his arrest.
However the Taiwan Association for Human Rights said the Chinese government should prove that Lee is not under arrest, citing the incident of five Hong Kong booksellers who went missing in 2015 and resurfaced in detention on the mainland