A Hong Kong publisher whose disappearance from the city raised fears he was abducted by Chinese officials has met his wife on the mainland and is reportedly "in good spirits", Hong Kong police said Sunday.

Lee Bo was the fifth employee of a Hong Kong publishing house known for producing salacious titles critical of Chinese leaders to have gone missing in recent months.

Lee, a British citizen, disappeared on December 30 in semi-autonomous Hong Kong, where mainland Chinese law enforcers are officially barred from operating.

Three others went missing in mainland China while Gui Minhai, who has Swedish nationality, disappeared in Thailand.

Hong Kong police said early Sunday that Lee, 65, had met his wife Sophie Choi at an unspecified location in China the previous day.

"Police have received notification from Lee Bo's wife that she met with Lee Bo at a guesthouse in mainland China in the afternoon of January 23," a statement said.

"Mrs Lee said Lee Bo is healthy and in good spirits. Now he is assisting an investigation as a witness," the statement said without elaborating.

There was no immigration record of Lee having left Hong Kong last month. Activists, local media and even pro-Beijing politicians in the city have expressed concern about the case.

They say any abduction would be a serious breach of the "One country, two systems" agreement, under which Hong Kong was returned to China in 1997 but was allowed to retain freedoms not available on the mainland.

Local media have published numerous letters purportedly written by Lee -- in which he said he entered the mainland voluntarily -- after his wife retracted her report to police listing her husband as missing.

But human rights activists said the developments smacked of "intimidation" tactics by China.

Hong Kong's Sing Tao newspaper published a photo on Sunday of Lee and his wife, as well as a letter said to have been written by him, in which he said he is "free and safe".

A weeping Gui Minhai appeared on Chinese state television last week, saying he was in China to "take legal responsibilities" for killing a college student in a car accident there 11 years ago.

His confession was rejected by activists, who described it as a "smokescreen".

Hong Kong is supposed to retain its self-governing and semi-autonomous system until 2047. But many fear Beijing is quietly imposing its authoritarian stamp on the city.