World Bulletin / News Desk

China said Wednesday it has detained a Taiwanese rights activist who went missing during a visit and is investigating him for suspected activities "endangering national security." 

The 42-year-old NGO worker "lost contact" on March 19 after he entered the southeastern Chinese city of Zhuhai from Macau, according to Taiwan's government. 

While confirming the probe, China's Taiwan Affairs Office (TAO) gave no further details of Lee's allegedly illegal activities. 

"On suspicion of engaging in activities endangering national security, the investigation is being handled according to judicial procedures," TAO spokesman Ma Xiaoguang told reporters. 

Lee is in good physical condition, he said. 

Ma added that Taiwanese travelling to the mainland for "normal activities" should not worry about their legal rights.  

"We will not place personal restrictions on Taiwan compatriots without reason," he said.

Human rights groups and Lee's wife called for his immediate release and for the Chinese government to disclose where he is being held. 

"A Taiwanese who entered China legally should not be arrested and detained at will for nine days without notifying relatives or legal counsel," they said in a joint statement Wednesday. 

Amnesty International said Lee's detention "on vague national security grounds raises fears the authorities are broadening their attack against those carrying out legitimate activism".

It said in a statement a new law targeting foreign NGOs came into force in January, giving authorities "virtually unchecked powers to target NGOs, restrict their activities, and ultimately stifle civil society".

Lee, a former employee of Taiwan's now-ruling Democratic Progressive Party (DPP), 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. 

Lee, who works for a community college in Taipei, has long supported civil society organisations and activists in China, according to Amnesty International. 

It said he went there this time to arrange for medical treatment for his mother-in-law. 

Ties between China and Taiwan have worsened since DPP President Tsai Ing-wen took office last May.  Beijing deeply distrusts her party, which is traditionally pro-independence. 

Taiwan has been self-ruled since 1949 following a civil war on the mainland but has never formally declared independence. 

China still claims the island as part of its territory and does not rule out military force to reclaim it.