WASHINGTON: US President Donald Trump reignited his trade war by threatening more than a dozen countries with higher tariffs on Monday – but then said he may be flexible on his new August deadline to reach deals.

Trump sent letters to trading partners including key US allies Japan and South Korea, announcing that duties he had suspended in April would snap back even more steeply in three weeks.

Tokyo and Seoul would be hit with 25 per cent tariffs on their goods, he wrote. Countries including Indonesia, Bangladesh, Thailand, South Africa and Malaysia were slapped with duties ranging from 25 per cent to 40 per cent.

But in a move that will cause fresh uncertainty in a global economy already unsettled by his tariffs, the 79-year-old once again left the countries room to negotiate a deal.

"I would say firm, but not 100 per cent firm," Trump told reporters at a dinner with visiting Israeli Prime Minister Benjamin Netanyahu when asked if the August 1 deadline was firm.

Pressed on whether the letters were his final offer, Trump replied: "I would say final – but if they call with a different offer, and I like it, then we'll do it."

The US president had unveiled sweeping tariffs on imports on what he called "Liberation Day" on April 2, including a baseline 10 per cent tariff on all countries.

But he quickly suspended all tariffs above 10 per cent for 90 days following turmoil in the markets.

They were due to kick back in on Wednesday and Trump sent the letters in advance of that deadline.

Trump's near-identically worded letters to Japanese and South Korean leaders said he would impose 25 per cent tariffs as their trading relationships with Washington were "unfortunately, far from Reciprocal."

He warned of further escalation if there was retaliation against the levies.

But Trump on Monday also signed an order formally extending the Wednesday deadline, postponing it to August 1.

The new August date effectively marks a further delay – and Trump's latest comments threaten to compound the uncertainty over when the deadline really is.

According to letters posted to Trump's Truth Social platform, products from Indonesia will face a 32 per cent tariff, while the level for Bangladesh is 35 per cent and Thailand, 36 per cent.

Most countries receiving letters so far had duties similar or unchanged from rates threatened in April, although some like Laos and Cambodia saw notably lower levels.

The Trump administration is under pressure to show results after promising "90 deals in 90 days."

So far only two firm deals have emerged, with Britain and Vietnam, plus an agreement to dial back super-high tit-for-tat tariffs with China.

Japan's Prime Minister Shigeru Ishiba said Sunday that he "won't easily compromise" in trade talks with Washington.

Asked why Trump opted to start with Japan and South Korea, Leavitt said: "It's the President's prerogative, and those are the countries he chose."

"This announcement will send a chilling message to others," said Asia Society Policy Institute Vice President Wendy Cutler, referring to Trump's initial letters to Tokyo and Seoul.

"Both have been close partners on economic security matters," she said, adding that companies from Japan and South Korea have made "significant manufacturing investments in the US in recent years."

US Treasury Secretary Scott Bessent said Monday that there would be more deals coming up: "We are going to have several announcements in the next 48 hours."

Major US stock indexes fell from records Monday on Trump's fresh threats. The Nasdaq tumbled 0.9 per cent and the S&P 500 lost 0.8 per cent.

Trump has also threatened an extra 10 per cent tariff on countries aligning themselves with the emerging BRICS nations, accusing them of "Anti-American policies" after they slammed his duties at a summit.

But partners are still rushing to avert Trump's tariffs altogether.

The European Commission said EU chief Ursula von der Leyen had a "good exchange" with Trump on trade when the pair spoke Sunday.