Hong Kong has seen continual erosions to the freedoms and autonomy it was promised by Beijing, which has also extended its repression of dissidents far beyond its borders, the U.S. Congressional-Executive Commission on China (CECC) said in an annual report.
The ruling Chinese Communist Party under President Xi Jinping has "run roughshod" over human rights, with China extending its crackdown on dissent far beyond its national borders, the report said.
CECC co-chairman Senator Marco Rubio said: "Beijing has become increasingly brazen in exerting its extraterritorial reach in the past year, as evidenced by the outrageous abductions of the Hong Kong booksellers."
Causeway Bay Books store manager and British passport-holder Lee Bo, 65, went missing from his workplace in Hong Kong on Dec. 30, 2015, while publisher Gui Minhai was detained at his holiday home in Thailand several weeks earlier.
The group's general manager Lui Bo (also spelled Lui Por), and colleagues Cheung Chi-ping and Lam Wing-kei were also all detained under opaque circumstances.
The five, all of whom are permanent Hong Kong residents, were accused of selling "banned books" to customers across the internal border in mainland China.
The CECC report also cited the barring of candidates in last month's Legislative Council (LegCo) elections if they were deemed to support the idea of independence for the former British colony, a notion that Chinese and Hong Kong government officials say is unconstitutional.
Political analyst Joseph Cheng of the campaign group Power for Democracy said the administration of Hong Kong chief executive Leung Chun-ying has also extended harsher and harsher treatment to its opponents in recent years.
"There are constant challenges to the notion of 'one country, two systems,' and now the fear is that not only will Hong Kong not have full democracy, we are now seeing an attack on our core values," Cheng said.
In August, a Hong Kong district court sentenced former student protest leader Joshua Wong to community service on a charge of unlawful assembly, alongside two other activists who were handed noncustodial sentences.
The Hong Kong government later requested a jail sentence for Wong and two fellow activists, although the request was turned down by the judge.
Cheng also said Beijing seems less and less willing to hear any criticism of its rights record from overseas.
Hong Kong Human Rights Monitor director Law Yuk-kai said the Hong Kong government appears less and less interested in maintaining the fiction of "one country, two systems."
"The invisible hand of China extends into every corner," Law said. "There is less and less room for Hong Kong's autonomy under 'one country, two systems' nowadays."
"At least for political dissidents, the past year has been a nightmare."
Law said most ordinary citizens in Hong Kong only have the city's traditional freedoms of speech, association and judicial independence to protect them.
"If the 'one country, two systems' idea disintegrates, there will be no protection for ordinary people, no power upon which we can call to enforce it," he said.
City's status at risk
He said China has little to lose by allowing it to disintegrate, but that the changes could affect the city's status as a world financial and business hub.
"Hong Kong will take the most harm ... because it's not just about our rights, but about the way we live our lives, our economy, our society," Law said. "All of that makes life easier for the international community."
Earlier this week, Thai immigration police detained Joshua Wong at the airport for 12 hours, denying him entry in a move that was widely believed to be at the request of Beijing.
Wong was told he was "blacklisted," and was escorted onto a plane back to Hong Kong, where he hit out at the Hong Kong government for its lack of response on his return.
Wong's denial of entry follows the repatriation of a number of Chinese dissidents and rights activists from Thailand after they were granted political refugee status by the United Nations.
The CECC meanwhile called for a stronger line from Washington on human rights.
"President Xi Jinping has run roughshod over human rights and the U.S. government has only responded tepidly," it said in a statement on its website.
"It is time to recognize that the economic engagement strategy has failed ... A new U.S. policy approach that champions individual liberties is owed to the thousands of suffering prisoners of conscience in China," it said.
Reported by Lee Lai for RFA's Cantonese Service. Translated and written in English by Luisetta Mudie.