WASHINGTON (AFP) - President Donald Trump on Wednesday (Aug 19) called arrested Hong Kong media mogul Jimmy Lai a "brave man" and expressed his support for the pro-democracy advocate who was briefly detained last week.

"I sent him best wishes," Trump told reporters at the White House.

Lai was among 10 people detained on August 10 under a sweeping security law imposed by Beijing, as around 200 police officers searched the newsroom of his Apple Daily tabloid, which is unapologetically critical of China.

The 71-year-old was cheered by staff and handed a bouquet as he returned to the newsroom two days later following a late-night release on bail after 40 hours in custody.

During the White House briefing, Trump called Lai "certainly a brave man."

Beijing's crackdown has provoked outrage in the West and deepened fears for millions who last year took to Hong Kong's streets to protest China's tightening grip on the city.

China has declared it will have jurisdiction over especially serious national security crimes, toppling the legal firewall between the mainland's Communist Party-controlled courts and Hong Kong's independent judiciary.

US authorities earlier in the day formally notified Hong Kong that the United States has withdrawn from three bilateral deals with the semi-autonomous Chinese city on extradition and taxation.

The announcement follows Trump's decision to revoke Hong Kong's preferential trade status.

In July, the president signed an executive order stipulating that the financial hub lacked the autonomy needed to justify special treatment as compared with China.