A Taiwanese publisher reported missing while visiting Shanghai is under investigation for suspected national security crimes, Chinese authorities said Wednesday.

Li Yanhe, the editor-in-chief of Gusa Publishing, is “under investigation by national security organs on suspicion of engaging in activities endangering national security,” Zhu Fenglian, spokeswoman for Beijing’s Taiwan Affairs Office, told a press conference.

“The relevant parties will protect (Li’s) legitimate rights and interests in accordance with the law,” she said.

Beijing has ramped up pressure on Taiwan since the 2016 election of President Tsai Ing-wen —  who regards the island as an independent nation and not part of “one China” — including by arresting several Taiwanese nationals.

Beijing’s confirmation of Li’s detention comes a day after Chinese authorities formally pressed secession charges against Taiwanese activist Yang Chih-yuan, leader of a minor political party advocating for the island’s formal independence.

Chiu Tai-san, head of the Mainland Affairs Council, Taiwan’s top policy-making body on China, warned on Wednesday of “long-arm jurisdiction” by Beijing.

“There is the smell of intimidation in this and it’s a show of suppression by them,” Chiu said at a parliament session, adding that Taiwanese authorities were “offering necessary assistance” to Li and Yang’s families.

Local media also quoted Chiu as saying that Li’s mother, sister and wife have been “warned” by Chinese authorities, without elaborating.

China claims self-ruled Taiwan as its territory and has vowed to seize it one day, by force if necessary.