Updated 5.15pm

US President Donald Trump said on Friday that he had saved Hong Kong from being destroyed by persuading Chinese President Xi Jinping to hold off on sending in troops to crush its pro-democracy movement.

"If it weren't for me, Hong Kong would have been obliterated in 14 minutes," Trump said in a scattershot early morning interview with Fox News.

Trump's comments come as he mulls signing congressionally-approved legislation in support of the pro-democracy activists - or bow to Beijing's threats of retaliation if the laws pass.

Asked whether he would veto the legislation, green-lit by an overwhelming margin in Congress on Wednesday, Trump equivocated.

"I'll tell you we have to stand with Hong Kong but I'm also standing with President Xi. He is a friend of mine he is an incredible guy," Trump said.

"I would like to see them work it out. We have to see them work it out," he added.

Trump cast his relationship with Xi as the bulwark keeping China from moving against the pro-democracy movement that has rocked Hong Kong during almost six months of increasingly violent protests.

He added that a "million soldiers standing outside of Hong Kong are not going only because I asked him: 'Please don't do that. You will be making a big mistake. It will have a tremendous negative impact on the trade deal.'"

Trump acknowledged that the tension over the former British colony - handed back to China in 1997 - has complicated efforts to strike a trade deal with Beijing, a source of economic uncertainty as Washington heads into an election year.

US and Chinese trade negotiators are "potentially very close" to a deal, he said.