HONG KONG (Reuters) - A Hong Kong-based Chinese official expressed concern on Friday at the disappearance of one of five missing people linked to a local publisher of books critical of Beijing's leadership but warned investigations were "complicated".

The comments were the most detailed yet from a Chinese official on the case that has alarmed many in Hong Kong and sparked a string of protests.

Wang Zhenmin, the recently appointed legal affairs chief of Beijing's office in Hong Kong, said that no Chinese law enforcers could take action in the city under the "one country, two systems" formula that govern Hong Kong's relations with Beijing.

The five men are widely thought to have been taken by agents working for the mainland in a breach of the wide freedoms and autonomy Hong Kong was promised as part of its handover from British to Chinese rule in 1997.

"We are very concerned about the legal case...like you," he told a university conference, acknowledging Hong Kong government investigations and its formal requests for explanations from mainland authorities.

"To investigate cases like this is very complicated. It takes time to find eventual truth."

Wang had been asked about Lee Bo, a shareholder of Causeway Bay Books and a British passport holder, who went missing from Hong Kong in late December. His wife has withdrawn a missing persons report saying the 65-year-old traveled to China voluntarily to assist in an investigation.

Four associates of Lee have also gone missing since late last year, including a Swedish national.

Britain, Sweden and the United States have expressed concern at the disappearances, which come amid growing fears of Beijing's meddling in routine affairs of the global financial hub.

Wang, who was recently a law dean at Tsinghua University in Beijing, said that it was "very, very clear" from Hong Kong's mini-constitution, the Basic Law, that mainland law enforcers could not "do such things" within the borders of the global financial hub.

Story continues

"I don't know the truth, like you, at the moment but I think the legal issue is very clear."

In a rare public appearance from a locally based mainland official, Wang was speaking at a conference on Chinese legal and political issues at the University of Hong Kong.

Hong Kong's leader Leung Chun-ying confirmed on Friday that his government had yet to receive a response from Beijing, despite inquiries on a number of official fronts.

(Reporting by Clare Baldwin; Written by Greg Torode; Edited by Nick Macfie)