The State Department said the success of Hong Kong, which was given a large degree of autonomy when British colonial rule ended in 1997, was due to its unique status.
"The United States... admires the city's outstanding achievements, which are the result of its high degree of autonomy, its economic and personal freedoms, and its respect for rule of law," said State Department spokeswoman Heather Nauert.
But while Washington valued its "excellent cooperation" with Hong Kong's government it "remains concerned about any infringements of civil liberties in Hong Kong, including intrusions on press freedoms," Nauert added in a statement.
Recent incidents in Hong Kong -- including the disappearance of five publishers who were known for salacious titles about the Chinese leadership -- have sparked fears that Beijing is choking the city's freedoms.
Last year, the city's High Court disqualified two democratically elected pro-independence lawmakers from taking their seats after they added expletives and used derogatory terms for China when taking their oaths.
The court's move was preempted by an earlier intervention from Beijing which said they should not be allowed to join parliament.
Mass protests in Hong Kong in 2014 demanding more democratic reform failed to wring any concessions from the authorities, leading to an increase in calls for self-determination or even full independence.
High-profile pro-democracy campaigners including Joshua Wong and young legislator Nathan Law were arrested Wednesday night after staging a protest outside a Hong Kong convention center that will host some of the anniversary events this weekend.
The State Department said the US "support(s) the further development of Hong Kong's democratic systems, including the implementation of universal suffrage in accordance with the Basic Law" -- a reference to its de facto constitution.
Hong Kong was handed back to China by colonial ruler Britain in 1997 under a "one country, two systems" formula designed to protect its freedoms and way of life.
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