Hong Kong's leader said he was "very concerned" Monday over the disappearance of five booksellers known for publications critical of the Chinese government after a prominent lawmaker accused mainland security officers of kidnapping the men.

The booksellers all worked for the same Hong Kong-based publishing house and are feared to have been detained by Chinese authorities, adding to growing unease that freedoms in the semi-autonomous city are being eroded.

Under its mini-constitution, Hong Kong enjoys freedom of speech and Chinese law enforcers have no right to operate in the city.

"I and related government departments are very concerned. The government cares very much about Hong Kong residents' rights and safety," Leung Chun-ying told reporters, saying it would be 'unacceptable' if mainland law enforcers were operating in Hong Kong.

"Only legal enforcement agencies in Hong Kong have the legal authority to enforce laws in Hong Kong," Leung said.

"If mainland law enforcement personnel enforce the law in Hong Kong, it is unacceptable because it is against the Basic Law (the city's constitution)."

Democratic legislator Albert Ho said Sunday he believed the men had been kidnapped by Chinese security officers.

At a regular briefing Monday, Beijing's foreign ministry spokeswoman Hua Chunying said: "I'm not aware of the situation; I have nothing to offer," when asked about the latest bookseller to disappear, Lee Bo, who went missing last week.

But an editorial in the Global Times newspaper, close to China's Communist Party, accused the bookstore run by the missing men of selling publications containing "maliciously fabricated content".

"These books spread to the mainland by various means, becoming a source of political rumours, and creating negative effects," it said.

"Although the... bookstore is based in Hong Kong, it maintains itself by causing trouble in the mainland."

The editorial, signed by Shan Renping, a pen name for the newspaper's editor Hu Xijin, said a "handful" of Hong Kongers were launching "political attacks".

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"In the era of the Internet, their impact is not limited to Hong Kong, but also leaks into the mainland, and becomes a genuine problem facing the country," it said.

When Leung was asked Monday whether he thought the men had been taken to the mainland, he said there was "no indication" and appealed for anyone with information to come forward.

- Pressure on leader -

Opponents criticised unpopular Leung, who is considered close to Beijing and a hate figure for Hong Kong's pro-democracy movement.

"The Hong Kong government and Leung Chun-ying should express to the top level on the mainland Hong Kong people's concern, instead of awaiting a reply," said pro-democracy lawmaker Lee Cheuk-yan.

Acting secretary for security John Lee said Sunday that Hong Kong police had made enquiries to their mainland counterparts and were yet to hear back, according to local media.

Lawmakers including Hong Kong's pro-Beijing former security chief Regina Ip have urged the government to investigate.

All five men worked for publishing firm Mighty Current, which is rumoured to have been about to launch a book on Chinese President Xi Jinping's former girlfriend.

The latest employee to disappear was 65-year-old Lee, last seen in Hong Kong Wednesday.

His wife said he had told her he was "assisting in an investigation" in a call made after he failed to come home for dinner Wednesday night.

She reported him missing to police on Friday.

Hong Kong police are investigating the disappearance of Lee and of three co-workers who are believed to have gone missing in Shenzhen.

The fifth, a Swedish national, was reported to have disappeared in Thailand.

Sweden's embassies in Bangkok and Beijing are reportedly investigating his disappearance.

Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and enjoys freedoms unseen on the mainland.