Chinese police have said for the first time that they are holding three Hong Kong booksellers who went missing on the mainland last year, sparking accusations that Beijing has "total contempt" for the law.

The admission confirms what many in the quasi-independent territory have suspected, and will reinforce fears that rights guaranteed under the principle of "One Country, Two Systems" are being eroded.

The three men all work for the Mighty Current publishing house, based in Hong Kong and known for salacious titles critical of the Chinese government.

Five booksellers from the firm have disappeared since October. All have now turned up in China, drawing international criticism.

Campaign group Amnesty International said Friday Chinese authorities had "total contempt for due process and the rule of law".

"The Chinese authorities need to end their smoke and mirrors strategy and come clean with a full and proper explanation," said Amnesty's China researcher William Nee.

Washington called on Beijing Monday to explain the disappearances, with a State Department spokesman saying the incidents "raise serious questions about China's commitment to Hong Kong's autonomy".

Booksellers Lui Por, Cheung Chi-ping and Lam Wing-kee disappeared in southern mainland China in October.

A fourth missing member of the company, Gui Minhai, a Swedish national, was paraded weeping on Chinese state television in January, where he said he had turned himself in for a fatal driving accident 11 years ago.

Gui had failed to return to Hong Kong from a holiday in Thailand in October.

In a letter to Hong Kong police, the Interpol Guangdong Liaison Office, part of the southern Chinese province's public security department, said the three men being held "were suspected to be involved in a case relating to a person named Gui, and were involved in illegal activities on the mainland."

"Criminal compulsory measures were imposed on them and they were under investigation," said the letter, released by Hong Kong police late Thursday.

- 'Let them go home' -

Enclosed was also a letter from the fifth missing bookseller, Lee Bo, Hong Kong police said.

Lee's case has sparked the strongest backlash as he was the only one of the men to have disappeared while in Hong Kong.

Lee was last seen at a book warehouse in his home city in December.

Lawmakers and activists have accused mainland authorities of snatching Lee from the city, contravening Hong Kong's laws which do not allow Chinese police to operate within the territory.

The new letter from Lee said Chinese authorities had told him Hong Kong police wanted to meet with him on the mainland, according to the Hong Kong police statement.

"He stated that he did not need to meet with police at the moment. He would contact police should he need to meet with police," the statement said.

It added that Lee's wife had confirmed the letter was in her husband's handwriting.

Some supporters of the booksellers believe they were targeted over a new book they were about to publish on the love life of Chinese leader Xi Jinping.

The author of the book, Chinese writer Xi Nuo, based in the US, urged China to release the men.

"They are not responsible for this. I'm responsible for this. I want to... tell the Chinese government: let the five guys go home," the author told the BBC.

Hong Kong police have asked the Guangdong authorities to "assist in following up the situations" of the three booksellers revealed to be under investigation, and to reiterate to Lee that Hong Kong police wanted to meet with him, the statement said.

Hong Kong was handed back to China by Britain in 1997 and its freedoms and way of life are protected under a 50-year agreement.

The disappearances have added to already simmering tensions over the erosion of those freedoms after attacks on journalists, increasing self-censorship and accusations of interference in the city's education institutions.