A Taiwanese man who went missing last month after crossing into China from Hong Kong is being investigated for activities that "endanger state security", Beijing said Wednesday.
The investigation against Lee Meng-chu comes amid unrest in the semi-autonomous city of Hong Kong and persistent tensions between China and the self-ruled democratic island of Taiwan.
Lee has not been heard from since he entered the border city of Shenzhen in mainland China on August 19, according to Taiwan's Mainland Affairs Council.
But in a press briefing on Wednesday, the mainland Taiwan Affairs Office confirmed that Lee was under investigation.
"Lee Meng-chu is being investigated according to the law because he is suspected of engaging in criminal activities that endanger state security," said Ma Xiaoguang, a spokesman for the mainland Taiwan Affairs office in Beijing.
Taipei called for Beijing to "immediately and fully explain" why and where Lee was being detained.
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Taiwan has been run as a de facto independent nation for the last seven decades but Beijing sees it as part of its territory awaiting reunification, by force if necessary.
Relations between the Communist-led mainland and the democratic island are frequently tense.
Taiwan's presidential office said in a statement it was "very concerned about our citizen Lee Meng-chu who has been detained by China".
Taipei said China should arrange for his family and a lawyer to visit to "ensure the legal rights he is entitled to".
A government spokesman said that Lee had been reachable while in Hong Kong, but his family lost contact with him after he entered mainland China.
News of Lee's detention comes at a sensitive time for China, with Hong Kong plunged into months of civil unrest.
Since the demonstrations, Chinese authorities have increased their inspections at the border with the mainland.
Last month a British consulate employee was detained in China for two weeks amid strained relations over what Beijing calls London's "interference" in the pro-democracy protests.
Beijing has slammed Taiwan for offering asylum to Hong Kong people facing prosecution for involvement in anti-government protests, telling the island's leaders to "stop meddling" in the territory's affairs.
Taiwan's ruling Democratic Progressive Party (DPP) said China's actions would "spark anger and unease in Taiwan's society and make our people doubt their personal safety and freedom when going to China.