HONG KONG - Hong Kong Chief Executive Leung Chun Ying pledged on Monday (June 20) to take up the case of the alleged kidnapping and illegal detention of a Hong Kong bookseller with Beijing, media reports said.

He said at a press conference 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 for more than eight months.

Mr Leung added that "if necessary", he would send government officials to the mainland to follow up on the matter, Radio Television Hong Kong and South China Morning Post reported.

"Today we will write to the central government to immediately express Hong Kong people's concerns on this case," said Leung.

"There's room for review and possibly, improvement, in the notification system," he added, referring to an arrangement between Hong Kong and Beijing that requires mainland authorities to notify the city's government of the detention of Hong Kong residents on the mainland.

Mr Leung, who returned to Hong Kong on Monday after a nine-day holiday, was speaking publicly for the first time since Lam surfaced last week and gave an explosive account of what happened during the eight months since he vanished during a visit to Shenzhen.

Mr Lam and four of his colleagues at publishing firm Mighty Currents and Causeway Bay Books store, which specialise in salacious gossip about China's leaders, went missing late last year.

Mr Lam said he had been accosted by 11 people last October after crossing the border into the southern city of Shenzhen and was taken to the eastern city of Ningbo. Two other colleagues, Mr Lui Por and Mr Cheung Chi Ping, were also taken into custody on the mainland in October. Another, Mr Lee Bo - also called Lee Po - vanished from Hong Kong in December, sparking allegations that he had been abducted.

Mr Gui Minhai, who holds a Swedish passport, disappeared from Thailand in October.

Police in the southern Chinese province of Guangdong only told their Hong Kong counterparts on Feb 4 - months after their disappearances - that Messrs Lam, Lui and Cheung were being probed in connection with Gui, author of some of the books.

By Mr Lam's account, he was allowed by mainland authorities to return to Hong Kong for two days last week on the condition that he returned to the mainland last Thursday.

But he decided to stay in Hong Kong and held a press conference where he told reporters about his and his colleagues' abductions and "rehearsed" confessions. He said he had spoken to Mr Lee when they were both detained in China, and that the latter told him that he had been kidnapped in Hong Kong.

He also said that a confession that he had made and which has been broadcast on Chinese television had been scripted, edited and supervised by a director.

The revelations have further deepened Hong Kongers' distrust of the central government. Some 6,000 protesters took to the streets on Saturday to demand freedom for Hong Kong.

Mr Leung said the police have tried to contact Lam and were waiting for his response. He said the police would provide him with assistance and ensure his personal safety.

But Mr Lee disputed Mr Lam's account in a Facebook post on Friday, saying he never told Lam how he got to the mainland and never said he was taken against his will. Mr Lui and Mr Cheung have disputed Mr Lam's account, according to interviews in Chinese newspaper Sing Tao Daily.

The case has fanned concerns about China's encroachment on the city's autonomy, guaranteed to the former British colony under the "One Country, Two Systems" principle when the former British colony was returned to China in 1997.