The new legislation by Beijing comes after demonstrations last year brought parts of Hong Kong to a standstill.
President Trump has said the US will «react strongly» to China if it pushes ahead with its plans to impose a new security law on Hong Kong.
Beijingmay try to gain more control over the former British colony and the move could see protests curbed after pro-democracy activists brought some areas to a standstill over several months last year.
During a tour of a factory in Detroit, Michigan, Mr Trump told reporters «nobody knows yet» what the details are of China’s plan.
But he said: «If it happens we’ll address that issue very strongly», without elaborating.
He said he would have a full statement on the proposals at an «appropriate time».
A senior Hong Kong government official said details about the move and its implementation were unclear.
But the city’s media has reported the legislation would ban foreign interference, terrorism and all seditious activities aimed at toppling the central government.
