Hong Kong and China have taken a step towards improving their relations by agreeing to inform each other within 14 days if they detain the other's residents for criminal investigation.

Both sides also agreed to provide more details about the offences of those detained and expand the notification channels to cover all law enforcement agencies, Hong Kong Chief Executive Leung Chun Ying said yesterday.

Mr Leung initiated the review after facing mounting pressure to address Hong Kongers' concern about the possibility that one of the missing booksellers of Causeway Bay Bookstore had been taken from Hong Kong by Chinese agents.

Mr Leung said the decision came after Hong Kong's justice and security chiefs spent hours meeting the mainland authorities, including the Ministry of Public Security and the Hong Kong and Macao Affairs Office of the State Council in Beijing, on Tuesday.

The Hong Kong delegation was also briefed on the case of Mr Lam Wing Kee who was accused of breaking mainland laws by selling political books that are banned in China, and Hong Kong police would continue with their inquiry, said Mr Leung.

On the same day the meeting was held in Beijing, Ningbo Public Security Bureau issued a statement warning Mr Lam that he could face tougher legal action for breaking his bail terms and failing to return to the mainland for further investigations.

Mr Lam, 61, one of the five booksellers who were reported missing last year, was allowed to return to Hong Kong last month, after being detained in Ningbo for eight months.

He later decided that he would not return to the mainland for fear of unfair treatment and gave an explosive account of his detention to the media in Hong Kong. Mr Lam also claimed that his business associate, Mr Lee Bo, was "taken against his will" from Hong Kong.

If true, "such action is a direct violation of the laws of Hong Kong and thereby has weakened the people's trust in the one country, two systems model", said criminal law expert Eric Seto, a partner of Morley Chow Seto in Hong Kong.

"This is the pressing concern of the people of Hong Kong, not the notification mechanism. Of course, we welcome a more effective notification system but it does not alleviate our concerns," he said.

Under the current administrative arrangement, there is no period set for the mainland to notify the Hong Kong police force of cases involving unnatural deaths.

Labelling the review as a "knee-jerk reaction" to Mr Lam's revelations, lawmaker Sin Chung Kai said he is doubtful of the Chinese authorities abiding by the agreement as the notification system has been in place since 2001.

Analyst Johnny Lau said the review provides a "gradual improvement" to the system, but it fails to address the bigger question of whether Chinese agents were operating in Hong Kong.

He told The Straits Times: "Even if the rules are amended now, Hong Kongers will still be concerned if the mainland authorities will abide by the rules."