The case of a bookseller who vanished from Hong Kong four months ago and mysteriously surfaced in mainland China was a "very unfortunate incident", a Beijing official said on Tuesday.

"That's (a) very unfortunate incident... no one wants such kind of case to happen here in Hong Kong and no one wants to see it happen again in future," said Wang Zhenmin, an official of China's liaison office in Hong Kong.

Wang, the head of the office's legal department, said he was happy Lee Bo had returned to Hong Kong and his normal life.

Five Hong Kong booksellers who worked for a local publishing house which specialised in gossipy works about Chinese leaders went missing in recent months.

Lee's case aroused particular concern because he disappeared in Hong Kong and did not pass through immigration at the border with mainland China, sparking fears he was illegally abducted by mainland agents.

Chinese police are barred from operating in semi-autonomous Hong Kong, which was guaranteed special freedoms under the deal which returned it to Beijing's sovereignty.

Three of the other four disappeared on the mainland and one in Thailand. They are now under criminal investigation on the mainland linked to trading illegal books.

Lee himself has insisted he visited the mainland on his own accord to assist in an investigation and is a free man.

Wang, speaking at the Foreign Correspondents' Club, did not clarify detail of the case, saying he was not familiar with the facts.

But he added that China would uphold the "One country, two systems" arrangement under which the city was returned by Britain in 1997.

"Only Hong Kong government and agencies can enforce the law within the legal jurisdiction of Hong Kong. Only Hong Kong law enforcement agencies can enforce laws. This is very clear and this is protected under the Basic Law and One country two systems," Wang said, referring to the city's mini-constitution.

His comments were a departure from Beijing's earlier rhetoric. Officials previously said the case was a domestic issue that should not be dealt with internationally.

Britain and other nations spoke out about Lee's case. Britain said it believed he was "involuntarily removed to the mainland" in what it called a "serious breach" of the handover agreement.

The handover deal allows Hong Kong to keep its special rights and freedoms for 50 years, but there are fears that such freedoms are being eroded.