China says it is investigating a Taiwan-based publisher for allegedly "endangering national security", in another move against Taiwan-linked individuals on the Chinese mainland.
Li Yanhe, who uses the pen name Fucha, published books critical of Beijing.
Beijing's confirmation of Mr Li's detention ends days of speculation about his whereabouts.
Taiwan has criticised China's "arbitrary arrests" of Taiwan residents as human rights breaches.
Confirmation of the publisher's arrest came a day after China said it would prosecute the founder of a pro-Taiwanese independence party for alleged secession.
Mr Li, the book publisher, was born in China and relocated to Taiwan in 2009. There, he set up Gusa Press, which has published books that are critical of Beijing.
Friends say Mr Li had arrived in China earlier this month to visit relatives and to take part in Qing Ming, the annual Chinese tomb-sweeping festival.
Reports that he was missing emerged about a week ago. Zhu Fenglian from China's Taiwan Affairs Office said Mr Li's rights would be respected during the investigation.
Dozens of writers, scholars and activists have called for his release.
The Taiwan Foreign Correspondents Club on Monday called in China to "respect the freedom of the press that it enshrines in its constitution, and to release all unjustly imprisoned media workers".
Mr Li's case has been widely compared with the disappearance of five Hong Kong booksellers in 2015 from a shop known for selling works critical of China.
They eventually turned up in the custody of mainland Chinese authorities, amid an investigation into their publishing business.
China's focus on Taiwan-linked individuals appears to have increased in recent times.
On Tuesday, Chinese authorities said they had completed an investigation into Taiwanese National Party founder Yang Chih-yuan and had put him under formal arrest.
The 32-year-old had been in China for unknown reasons. In August, he was arrested in the eastern Chinese city of Wenzhou on suspicion of "separatism".
At the time his detention was linked to a Chinese crackdown on "separatists" amid tensions over the then US House Speaker Nancy Pelosi's trip to Taiwan.
Chinese state media reports said he had been "poisoned by thoughts of Taiwan independence secessionism for a long time" and been "actively scheming" to work towards formal statehood for Taiwan.
Through his party, he had "actively planned and implemented" a series of events to "seek independence and reject unification".
State media also said that Mr Yang had advocated for Hong Kong's independence with other "separatist" forces.
The issue of "secession" is sensitive in China. Beijing sees Taiwan as a breakaway province that will eventually be brought under its control.
On Tuesday, Taiwan's Mainland Affairs Council, a cabinet-level agency responsible for cross-strait relations, said it had repeatedly asked for Mr Yang to be released since his arrest in August, but had not received a positive response from China.
Also this month, two Taiwan-based reporters for Taiwan's EBC News were detained by Chinese authorities while filming military exercises in Pingtan county in China's Fujian province.
Local media have identified the reporters by the last names, Huang and Li. They are said to be safe and maintain daily contact with a director at their company.
Taiwanese authorities on Tuesday reminded their residents to "assess relevant risks" before visiting China.
"You should realise that mainland China is accustomed to random violations of personal safety," a MAC spokesperson said.
Source: BBC