Hong Kong's leader says he'll raise concerns with Beijing over the case of a bookseller secretly detained for months in mainland China.

Chief Executive Leung Chun-ying also said Monday officials would review a notification system that's supposed to alert the Hong Kong government if one of its residents is detained by mainland authorities for illegal activity.

Leung told reporters he "attaches great importance" to the case of Lam Wing-kee, one of five booksellers whose disappearance raised international concern over fears Beijing is eroding Hong Kong's wide autonomy.

The bookseller returned to Hong Kong last week and spoke publicly about his ordeal.

In Leung's first public comments on the case, he said he would write to Beijing to express Hong Kong residents' "high level of concern" over Lam's disappearance.