President Donald Trump tweeted Tuesday that U.S. intelligence believes that the Chinese government is moving troops to its border with Hong Kong.
Trump also tweeted that "Everyone should be calm and safe!" He provided no additional details.
Riot police briefly clashed with pro-democracy protesters outside the Hong Kong Airport for the second day on Tuesday though calm returned to the airport in the early hours of Wednesday.
The airport disruptions are an escalation of a summer of demonstrations against what some in Hong Kong see as an increasing erosion of freedoms. The protests have in recent weeks turned to focus on police brutality against demonstrators.
Earlier Tuesday, Trump called the Hong Kong protests a "very tricky situation," saying he hopes no one is injured or killed.