So, was he already contemplating the tariff pause when he made that post?

Donald Trump stocks were wavering between gains and losses. But that was about to change.

"THIS IS A GREAT TIME TO BUY!!! DJT," he wrote on his social media platform Truth Social at 9.37 am on Wednesday.

Less than four hours later, the US president announced a 90-day pause on nearly all his tariffs.

Stocks soared on the news, closing up 9.5 per cent by the end of trading. The market, measured by the S&P 500, gained back about $US4 trillion ($A6.5 trillion), or 70 per cent, of the value it had lost over the previous four trading days.

It was a prescient call by the president. Maybe too prescient.

"He's loving this, this control over markets, but he better be careful," said Trump critic and former White House ethics lawyer Richard Painter, noting that securities law prohibits trading on insider information or helping others do so.

"The people who bought when they saw that post made a lot of money."

The question is, was Trump already contemplating the tariff pause when he made that post?

Asked about when he arrived at his decision, Trump gave a muddled answer.

"I would say this morning," he said.

"Over the last few days, I've been thinking about it. Fairly early this morning."

Asked for clarification on the timing in an email to the White House later, a spokesman didn't answer directly but defended Trump's post as part of his job.

"It is the responsibility of the president of the United States to reassure the markets and Americans about their economic security in the face of nonstop media fear mongering," wrote White House spokesman Kush Desai.

With AAP.