Donald Trump has said that China is doing very poorly as a result of the trade war and the "longer it goes on, the stronger we get".

Asserting that the United States is "having a very good discussion" on a trade deal with China, Trump told reporters at the Morristown airport in New Jersey that he will speak with Chinese President Xi Jinping "very soon".

On the protests in Hong Kong which have been going on since early June, the US president said that Xi can "humanely" solve the problem so that "everybody is happy".

"I think the longer the trade war goes on, the weaker China gets and the stronger we get. We're taking in massive amounts of money. Billions and billions of dollars. And I think the longer it goes, the stronger we get. I have a feeling it's going to go fairly short," he said on Thursday.