Trump warns China trade deal could take years

US President Donald Trump warned Tuesday that efforts to resolve a trade dispute could wait until after next November's US election.

"I have no deadline," Trump told reporters in London. "In some ways I like the idea of waiting until after the election for the China deal."

Trump's trade war with China and on-again off-again attempts to reach a deal have de-stabilised markets and stoked geo-political tensions.

As late as last week Trump boasted the he was in the "final throes" of negotiating "one of the most important deals in trade ever".

But Washington has since courted Chinese anger by expressing support for Hong Kong protesters, calling progress into doubt.

"I'm doing very well in a deal with China, if I want to make it," Trump said at a press conference ahead of the NATO summit.

"I don't think it's 'if they want to make it', it's 'if I want to make it'. And we'll see what happens.

"I don't know if I want to make it but you're going to find out pretty soon, it's going to surprise everybody," he said.