HONG KONG • A missing Hong Kong employee from a publisher of books critical of China was "assisting in an investigation", his wife said yesterday, as the police also probe the disappearance of his colleagues.

Mr Lee Bo, 65, went missing Wednesday night and is the fifth employee of Hong Kong-based publisher Mighty Current to disappear. The incident adds to growing unease that freedoms in the semi-autonomous Chinese city are being eroded, with fears the five men may have been detained by Chinese authorities.

"He said he wouldn't be back so soon and he was assisting in an investigation," Mr Lee's wife, Ms Sophie Choi, told Hong Kong's Cable Television, describing a call she had with him the night he failed to return home.

"I asked him if it was related to the case before. He said 'yes', regarding that case where a few others had gone missing."

The police said in a statement they were investigating the disappearance of Mr Lee and three of the other missing men.

They made no comment on the fifth man.

Hong Kong Chief Secretary Carrie Lam tried to reassure the public.

"The Hong Kong government cares about its people's well-being... (the) police are working on this case," she told reporters.

Ms Choi had said earlier she started looking for her husband on Wednesday night after he failed to return home for dinner and she reported him missing to the police on Friday.

He later called to say "everything was alright" from a number that did not belong to him and originated from the neighbouring Chinese city of Shenzhen.

The Hong Kong Journalists Association sent a letter to the Chinese Liaison Office, Beijing's representative office in the city, urging the authorities to reveal whether the men are in China.

AGENCE FRANCE-PRESSE