The United States called on China on Monday to clarify the status of five missing Hong Kong booksellers, saying the case raised serious questions about China's commitment to Hong Kong's autonomy under the "one country, two systems" framework
US State Department spokesman John Kirby said the United States was "deeply" concerned.
"We urge China to clarify the current status of all five individuals and the circumstances surrounding their disappearances and to allow them to return to their homes," he said in a regular news briefing.
The disappearances have prompted fears that mainland Chinese authorities may be using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.
Britain left Hong Kong to China under the agreement that its broad freedoms, way of life and celebrated legal system would remain unchanged for 50 years.Some of the former British colony's pro-democracy lawmakers, activists and residents believe mainland authorities are kidnapping critics to try to silence dissent.
"These cases ... raise serious questions about China's commitment to Hong Kong's autonomy under the one country, two systems framework, as well as its respect for the protection of universal human rights and fundamental freedoms," said Kirby.
The Chinese Foreign Ministry said on Tuesday it was "not proper" for the US to comment on China's domestic affairs in Beijing.
"Hong Kong residents have been fully entitled to freedoms and rights in accordance with law" since the territory’s return to China, the ministry said in a regular briefing.
One of the missing men, Lee Bo, 65, is a dual British and Chinese national and owner of a publisher and bookstore specialising in books critical of China's Communist Party leaders.
Four other booksellers are believed to be still in mainland detention, including Swedish national Gui Min-hai.
Leo Bo and Gui Min-hai were remoured to be preparing a book about the love life of President Xi Jinping.
Leo Bo disappeared in Hon Kong, where mainland law enforcers have no authority to operate in. The British government is still waiting for responses to its diplomatic requests for information and access to Lee, who disappeared from Hong Kong on Dec. 30.