Donald Trump has warned that all goods from Japan and South Korea will face tariffs of 25% from 1 August.
The announcement, via his Truth Social platform, marks the restart of the threatened "liberation day" escalation that was paused in April, for 90 days, to allow for negotiations to take place with all US trading partners.
The US president included copies of letters to the leaders of both Japan and South Korea informing them of the tariff rates. Those duties will come on top of sector-specific tariffs - such as 50% rates covering steel - which are already in force.
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He warned the rates could be adjusted "upward or downward, depending on our relationship with your country".
Country-specific tariffs had been due to take effect from Wednesday this week but Mr Trump had earlier revealed that nations would start to get letters instead, setting out the US position.
Duties would take effect from 1 August, without any subsequent deal being agreed, it was announced.
The letters sent to Japan and South Korea cited persistent trade imbalances for the rates and included the sentence: "We invite you to participate in the extraordinary Economy of the United States, the Number One Market in the World, by far."
He ended both letters by saying, "Thank you for your attention to this matter!"
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The European Union - the biggest single US trading partner - is among those set to get a letter in the coming days.
Mr Trump has also threatened an additional 10% tariff on any country aligning itself with the "anti-American policies" of BRICS nations - those are Brazil, Russia, India, China and South Africa and whose members also include Egypt, Ethiopia, Indonesia, Iran and the United Arab Emirates.
The UK, bar a massive shock U-turn, should be exempt.
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The country was the first to be granted a trade deal, of sorts, in May and the Trump administration has claimed many others had been offering concessions since the clock ticked down to 9 July.
The UK is not expected to face any changes to its current 10% rate due to the trade truce, which came into effect last week.
While UK-made cars aerospace products face no duties under a new quota arrangement, it still remains to be seen whether 25% tariffs on UK-produced steel and aluminium will be cancelled.
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They could, conceivably, even be raised to 50%, as is currently the case for America's other trading partners, because no agreement on eliminating the rate was reached when the government struck its deal in May.
It all amounts to more uncertainty for the UK steel sector.
A No 10 spokesman said on Monday: "Our work with the US continues to get this deal implemented as soon as possible.
"That will remove the 25% tariff on UK steel and aluminium, making us the only country in the world to have tariffs removed on these products.
"The US agreed to remove tariffs on these products as part of our agreement on 8 May. It reiterated that again at the G7 last month. The discussions continue, and will continue to do so."
China and Vietnam have also secured some US concessions.
The dollar strengthened but US stock markets lost ground in the wake of the letters to Japan and South Korea being made public, with the broad-based S&P 500 down by 1%.
Stock markets in both Japan and South Korea were closed for the day but US-traded shares of SK Telecom and LG Display were down 7.5% and 5.8% respectively.