HONG KONG (AFP) - Rights groups expressed concern on Thursday (Nov 19) after the apparent disappearance of four people who work for a Hong Kong publishing firm known for selling books critical of the Chinese government.

It is the latest incident to fuel growing unease in Hong Kong over the erosion of freedoms in the city, with fears that the four individuals have been detained by Chinese authorities.

Mr Gui Minhai, a Swedish national and co-owner of the Mighty Current publishing company, failed to return from a holiday in Thailand in October, local media reported.

The publishing company's general manager Lui Bo, an employee Cheung Jiping and bookstore manager Lam Wing-kei are also reportedly missing after disappearing in southern China last month.

One shareholder of Mighty Current, who gave his name as Mr Lee, told AFP the four were still missing on Thursday.

"I think (it has happened) probably because of publishing matters... political books banned on the mainland," Mr Lee said.

He added that Mr Gui's wife had received a call from her husband on Tuesday asking her "not to go to Thailand", where it is reported he has a holiday home.

"There seems to be a concerted effort from the mainland authorities to prevent Chinese political books from travelling from Hong Kong to China - we are concerned about that and the fact the disappearance may be somewhat related," said Ms Maya Wang, a China researcher for Human Rights Watch (HRW).

Another Hong Kong publisher, Mr Yao Wentian, due to release a dissident's book about Chinese President Xi Jinping, was reported to have been detained for almost three months in January last year.

In May 2014, Mr Yao, then 73, was sentenced to 10 years' jail for smuggling by a Chinese court.

HRW's Ms Wang also raised concern after rights groups said Thai authorities had deported dissidents back to China, although it is unclear if they played any role in Gui's case.

Amnesty International's China researcher William Nee said the publishers' case was "very worrying" for freedom of expression in Hong Kong.

When asked if it was investigating, Hong Kong's immigration department told AFP it had "received a request for assistance regarding a suspected missing Hong Kong resident and rendered practical assistance according to the wish of the family members", without giving further details.

Police also told AFP one of the four had been formally reported missing.

"On Nov 5, a 59-year-old female surnamed Cheng reported to police that she was unable to contact her husband, 59, surnamed Lam," Hong Kong police told AFP.

Ms Cheng later told police she had resumed communications with her husband, the spokesman said.

Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and enjoys the right to freedom of the press and publication, but there are fears those freedoms are under threat.

The Hong Kong Journalists Association said reporters faced an "unprecedented" number of assaults during last year's pro-democracy rallies.

Two men were also jailed for 19 years in August over a brutal knife attack on a veteran Hong Kong journalist.