The daughter of one of five Hong Kong booksellers who went missing late last year has called on US authorities to help end her father's "unofficial and illegal" detention in China.

Four booksellers working for a Hong Kong publishing house which specialised in gossipy works about Chinese leaders went missing from various locations in October, with another disappearing at the end of December.

They all reappeared eventually in mainland China.

The disappearances fuelled growing unease in Hong Kong over the erosion of freedoms in the semi-autonomous city, which was handed back to China from Britain in 1997.

"It has now been eight months since my father and his colleagues were taken into custody. I still haven't been told where he is, how he's been treated, or what his legal status is," Angela Gui said at a US congressional hearing Tuesday in Washington DC.

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

His "unofficial and illegal" detention "is especially shocking in light of the fact that my father held Swedish and only Swedish citizenship", she told the Congressional-Executive Commission on China.

Three other booksellers disappeared on the mainland and one went missing from Hong Kong itself.

Gui mysteriously re-emerged on state broadcaster CCTV in January, and said he had returned to China to "take legal responsibilities" for killing a college student in a car accident 11 years earlier.

In another televised confession in February, he said he tried to smuggle illegal books into China.

Angela Gui said she wanted "the US to take every opportunity to ask China for information on my father's status as well as urge that he be freed immediately".

Swedish authorities have said they were "quite concerned" about the incident and have called for more openness from Chinese authorities.

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

All four are under criminal investigation on the mainland.

The fifth bookseller, Lee Bo, has said he travelled to China to assist with the investigation and returned to Hong Kong in March.

The disappearance of Lee Bo, who went missing from Hong Kong, raised fears that Chinese security authorities were operating in Hong Kong in violation of the city's laws.

Britain and other nations have spoken out about Lee's case. Britain said it believed he was "involuntarily removed to the mainland" in what it called a "serious breach" of the handover agreement.

The handover deal allows Hong Kong to keep its special rights and freedoms for 50 years, but there are fears that such freedoms are being eroded.