President Donald Trump is poised to significantly ramp up pressure on China if a trade deal is not agreed upon soon, according to an outside adviser to the White House who called tariffs so far "low level."

In an interview in Hong Kong on Thursday with the South China Morning Post, Michael Pillsbury said Trump had been "remarkably restrained in the pressure he has brought to bear on China in the trade field."

"Does the president have options to escalate the trade war? Yes, the tariffs can be raised higher," he told the news outlet.

"These are low-level tariffs that could go to 50% or 100%," he said, adding that Trump's critics were wrong to assume he is "just bluffing" when he threatened an all-out trade war.

"There are other options involving the financial markets, Wall Street, you know, the president has a whole range of options," he said.

Pillsbury, who has advised the White House and previous Republican presidents going back the Reagan administration, stressed he was not speaking for the administration. Trump's tweets are the best guide to his thinking, he said.

"I believe President Trump uses social media, especially on China, to convey his thinking," he said. "So I reject the idea that I or anyone else is some kind of adviser to him on China.

"His focus is revealed frequently in the tweets that I think everybody should take very seriously as presidential statements."

Trade officials from both sides are expected to resume negotiations in Washington next month.

• Trump Trade-War Boomerang Hits Steelmakers as Prices, Profit Sag
• Study: Trade War Will Cost at Least $460 Per Family