Hong Kong bookseller Lee Bo, who went missing three months ago and surfaced in China, is already on his way back over the border after a brief return home, local media reported.

British citizen Lee, 65, returned to Hong Kong Thursday after disappearing in late December, in a case that has raised alarm over Beijing's tightening grip on the region.

But Lee crossed back into the mainland Friday, just a day after he arrived, according to local media who followed him to the border.

"It's a release with Chinese characteristics," China expert Willy Lam of the Chinese University of Hong Kong told AFP.

"The fact of the matter is that he has not really been fully released... he needs to report back to China," he said.

Lee is one of five Hong Kong booksellers who went "missing" in recent months -- the other four are now under criminal investigation on the mainland linked to trading illegal books in China.

The men all worked for the Mighty Current publishing house, which produced salacious titles about political intrigue and love affairs at the highest levels of Chinese politics.

Lee's case caused the greatest outcry because he was the only bookseller to disappear from Hong Kong, prompting accusations that Chinese law enforcement agents were operating in the semi-autonomous city, illegal under its constitution.

Three of the other booksellers went missing from southern mainland China and one from Thailand in October.

Lee had returned to Hong Kong Thursday, where he insisted a missing person case on him should be dropped and that he was a free man.

He told Hong Kong's pro-Beijing Phoenix TV Thursday that he "may need to return to the mainland multiple times to assist in the investigation".

Lee vowed not to sell "fabricated books", according to Chinese news portal thepaper.cn, and said he would no longer run Hong Kong's Causeway Bay bookstore, outlet for Mighty Current's titles, which remains shuttered.

"The homeland is very prosperous and formidable. I am very proud to be Chinese," it quoted him as saying.

- 'Hard to believe' -

Lee told reporters Friday he would return to China with his wife to pay respects to his ancestors. He has also said he would be prepared to bring his autistic son to the mainland, praising China's medical treatment as "advanced".

"Before, there was a time I was afraid to go back to the mainland, because I heard that some people had got into trouble because of these books," Lee said outside his Hong Kong apartment block Friday.

"The problems have been solved."

Asked whether he had been abducted from Hong Kong by security agents, Lee answered: "It's not convenient for me to say."

Smiling and laughing nervously, Lee was escorted into the back of a black people carrier. Soon after, he was spotted at the border.

Immigration officials refused to confirm whether Lee had crossed back over to China.

"It's now becoming a pattern, it really just makes it even harder to believe that the so-called released booksellers actually have freedom," said Amnesty International's China researcher William Nee.

Bookseller Gui Minhai, a Swedish citizen, confessed to trying to smuggle illegal books into China in a television interview in February.

Colleagues Cheung Chi-ping, Lui Por and Lam Wing-kee blamed the company's illegal book trade on Gui.

Cheung and Lui returned to Hong Kong earlier this month on bail, but are reported to have quickly gone back to the mainland.

The case has drawn international criticism, with Britain saying it believed Lee had been "involuntarily removed" to the mainland, in a "serious breach" of an agreement signed before Britain handed Hong Kong back to China in 1997, which protects the city's freedoms for 50 years.