The five residents are associated with Mighty Current Media and the Causeway Bay Bookstore and have disappeared since October from mainland China or Thailand.
"We continue to follow closely the developments of these cases," State Department Spokesman John Kirby told reporters.
"These cases, including two involving individuals holding European passports, 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," he said.