US President Donald Trump attends a meeting in Washington. File photo

US President Donald Trump said on Tuesday the United States was working on a strong response to China's planned national security legislation for Hong Kong and it would be announced before the end of the week.

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China's parliament is expected to approve a proposed security law that would reduce Hong Kong's separate legal status on Thursday, calling into question the special economic status the territory currently enjoys under US law.

At a White House news briefing, Trump was asked if he planned sanctions against China over Hong Kong and if he intended to put restrictions on visas for students and researchers from China.

Riot police guard detained anti-government protesters in the Causeway Bay district of Hong Kong on Wednesday. AP

"We're doing something now. I think you'll find it very interesting ... I'll be talking about it over the next couple of days," he replied.

Pressed if this would include sanctions, he said: "No it's something you're going to be hearing about ... before the end of the week, very powerfully I think."

Trump did not elaborate.

China responded to the comments with a warning it would retaliate against any new measures.

"We will not accept any foreign interference, and to the wrong actions of outside powers in interfering in Hong Kong, we will take necessary countermeasures to hit back," foreign ministry spokesman Zhao Lijian told a regular briefing.

"The issue ... is purely China's internal affair."

Reuters