Donald Trump was assailed on Tuesday for his hands-off approach to pro-democracy protests in Hong Kong, avoiding criticising Beijing even as he cited US intelligence reports of Chinese troops moving to the territory’s border.

Critics on both sides of the political spectrum accused the US president of abandoning longstanding US policy to support democratic movements and giving Beijing a green light to intervene in one of the world’s most important financial and trade centres, a semi-autonomous Chinese region.

As protesters battled police in Hong Kong’s airport on Tuesday, partially shutting down air traffic, Mr Trump appeared ambivalent, telling journalists the situation was "very tricky."

"I hope it works out for everybody including China. I hope it works out peacefully, nobody gets hurt, nobody gets killed," he said.

Shortly afterwards, Mr Trump tweeted that US intelligence "has informed us that the Chinese Government is moving troops to the Border with Hong Kong".

"Everyone should be calm and safe!" Mr Trump wrote, retweeting a video of military trucks in the Chinese city of Shenzhen.