Christian pro-democracy campaigner Joshua Wong has expressed his disappointment over US President Donald Trump's comments on the Hong Kong protests.

President Trump last week spoke of the protests as "riots", echoing the language from Beijing, and said the unrest was between Hong Kong and China to resolve, disappointing pro-democracy campaigners who had been hoping for American support.

"Something is probably happening with Hong Kong, because when you look at, you know, what's going on, they've had riots for a long period of time," Trump told reporters at the White House on Thursday.

Commenting on what China's attitude to the unrest might be, he said, "Somebody said that at some point they're going to want to stop that. But that's between Hong Kong and that's between China, because Hong Kong is a part of China."

Hundreds of thousands of Hong Kongers have been marching through the streets in the last few months to protest an extradition bill that would allow certain criminal suspects to be transferred to China for trial.

The protest has evolved, though, from a single issue to concerns about democracy more widely and the use of excessive force by police.

The Hong Kong garrison of the Chinese People's Liberation Army released an ominous video last week showing troops engaged in "anti-riot" drills. The pro-democracy camp has interpreted this video as a warning.

Wong, who has been a prominent figure in the pro-democracy movement since the 2014 Umbrella protests, urged Trump to back the bipartisan letter from US Senator Marco Rubio, who has been vocal in his condemnation of China.

The letter urges Trump to "condemn China's efforts to undermine Hong Kong's autonomy".

It calls the extradition bill "problematic" and warns that it "would allow for the people of Hong Kong and foreigners to be extradited to China, where the judicial system is often used as a tool of repression".

"Protesters have been met with violence, and Beijing has accused the US of fomenting the unrest. We therefore urge you to issue a strong statement condemning Beijing's efforts to undermine Hong Kong's autonomy, endanger US residents of Hong Kong, and its suspected use of hired thugs to suppress the protests by the people of Hong Kong," he said, referring to the Yuen Long attack on protesters which is widely rumored to have been carried out by Triad gangsters.

He continued: "Failure to respond to Beijing's threats will only encourage Chinese leaders to act with impunity.

"Hong Kong's governance is not China's internal affair. China must fully abide by the terms and the spirit of the 1984 Sino-British Joint Declaration, a legally binding treaty that guarantees Hong Kong's legal autonomy from the mainland.

"US interests are at stake if Hong Kong's autonomy is further eroded. The Chinese government's willingness to uphold its international commitments have a direct bearing on whether any agreement with Beijing, especially on trade, can ever be trusted."

The letter was also signed by Democratic Senator Ben Cardin, Democratic Representative Jim McGovern and Republican Representative Chris Smith.

In response to Trump's comments, Wong tweeted: "I hope@realDonaldTrump reevaluates his position on HK with what America stands for in mind, and to consider the bipartisan, bicameral letter sent to him today by@marcorubio, @BenCardinforMD, @RepChrisSmith and@RepMcGovern."

He is also urging British people to take their concerns to their MPs by signing the Stand With HK petition.

"HK's fundamental freedom, rights, rule of law and democracy are under attack. China breached the 1984 Sino-British Joint Declaration. Tell MPs, Britain must stand by its promises & take action," he said.