The Chief Executive, CY Leung, on Monday vowed to take up the matter of the alleged kidnapping of the Causeway Bay booksellers with the central government.
He said the Hong Kong government will write to Beijing to reflect the concern of the city's people over bookseller Lam Wing-kee's claim that he was kidnapped and kept in illegal detention.
Leung added that if necessary, he would send government officials to the mainland to follow up on the matter.
He also said a review would be carried out of the arrangement stipulated in a memorandum between Hong Kong and the central government that requires mainland authorities to notify the city's government about the detention of its residents on the mainland.
Leung went on to say that the police tried to contact Lam and were waiting for his response. He said the police would provide him with assistance and ensure his personal safety.
Lam has alleged that he was kidnapped and detained on the mainland for selling books critical of Beijing leaders.
There was no communication about his detention from the mainland authorities, raising charges that Beijing had completely ignored the "One Country, Two Systems" principle.
Lam also said his colleague Lee Bo was taken away from Hong Kong against his will. But Lee has denied this.