Hong Kong (CNN) Taiwan's ruling party has warned citizens against traveling to Hong Kong or mainland China, saying the situation is "severe," after a Taiwanese businessman was detained following a visit to the protest-wracked city.

Lee Meng-chu, 43, disappeared after he crossed over the border from Hong Kong to the mainland Chinese city of Shenzhen on August 19. He was reported missing by a friend, Chen Ya-lin, who was supposed to meet him at Jakarta airport two days later. Chen said Lee had attended a protest in Hong Kong before crossing the border.

On Wednesday a spokesman for China's Taiwan Affairs Office confirmed that Lee was being questioned for "criminal activities that could jeopardize China's national security." The spokesman did not elaborate on the specific accusations against Lee.

Hsueh Cheng-yi, a spokeswoman for Taiwan's ruling Democratic Progressive Party (DPP), said that the detention had led to fears in Taiwan over the safety of travelers heading to Hong Kong or mainland China.

"The situation inside Hong Kong and China is severe and travel should be reduced. If you have to go to these areas, you must pay close attention to your own safety, and keep your friends and relatives informed of your whereabouts," Hsueh said in a statement.

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