Washington called for Beijing to explain the disappearance of five Hong Kong booksellers Monday, with a State Department spokesman saying the incidents "raise serious questions about China's commitment to Hong Kong's autonomy".
The five, all affiliated with Hong Kong's Mighty Current publishing house, known for salacious titles critical of Beijing leaders, disappeared in recent months and are feared to have been detained in mainland China.
"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," State Department spokesman John Kirby told reporters in Washington.
The five include two citizens of European countries who went missing in Thailand and in Hong Kong, where mainland law enforcers have no authority to operate, raising fears of Chinese authorities extending their reach internationally.
Lee Bo, who has a British passport, and Swede Gui Minhai were both born in China and were rumoured to be preparing a tell-all book about the love life of President Xi Jinping.
Activists, local media and various politicians in semi-autonomous Hong Kong have expressed concern that the men may have been abducted, which would be a serious breach of the "One country, two systems" agreement, under which the city was returned to China in 1997 but retained freedoms not available on the mainland.
Some of the former British colony's pro-democracy lawmakers, activists and residents believe mainland authorities are kidnapping critics to try to silence dissent.
Lawmakers from Britain and the European Union have also spoken out on the disappearances, with Sweden's foreign minister saying the treatment of its citizens was "completely unacceptable".