US President Donald Trump said Tuesday he was ending trade preferences for Hong Kong and signed into law an act that authorizes sanctions on banks over China's clampdown in the city.

In a news conference driven by criticism of his domestic rivals, Trump declared himself to be tougher than ever on China including over its treatment of the financial capital.

"Hong Kong will now be treated the same as mainland China -- no special privileges, no special economic treatment and no export of sensitive technologies," Trump said in the White House Rose Garden.

"Their freedom has been taken away; their rights have been taken away," Trump added.

US President Donald Trump announces new action against China in a news conference in the Rose Garden of the White House
US President Donald Trump announces new action against China in a news conference in the Rose Garden of the White House AFP / JIM WATSON

"And with it goes Hong Kong, in my opinion, because it will no longer be able to compete with free markets. A lot of people will be leaving Hong Kong."

Trump said he had signed into law the Hong Kong Autonomy Act, which passed overwhelmingly in Congress as Beijing pushed through a tough national security law in Hong Kong.

The new US law authorizes sanctions against Chinese officials and Hong Kong police seen as impeding on the city's autonomy -- and, crucially, any banks that make significant transactions with them.

Lawmakers earlier said that some officials of the Trump administration had hesitated over the legislation because it reduces the president's ability to waive sanctions.

China's new law in Hong Kong criminalizes subversion among other offenses, sending a chill through the city that was promised autonomy before Britain handed it back in 1997.