President Donald Trump says the US will impose a 25 per cent tariff on imports from Japan and South Korea beginning August 1 as he unveiled the first two of what he has said will be a wave of letters to trading partners outlining the new levies they face.
More countries are expected to receive letters, according to the White House.
"If for any reason you decide to raise your Tariffs, then, whatever the number you choose to raise them by, will be added onto the 25 per cent that we charge," Trump said in letters to the leaders of the two Asian countries, which he posted on his Truth Social platform.
The rate for South Korea is the same that Trump initially announced on April 2, while the rate for Japan is one point higher than first announced.
Trump a week later capped all of the so-called reciprocal tariffs at 10 per cent until July 9 to allow for negotiations.
Only two agreements have so far been reached, with Britain and Vietnam.
US stocks fell in response, the latest market ruction since Trump unleashed a global trade war on his return to office in January.
His moves have repeatedly roiled financial markets and sent policymakers scrambling to protect their economies.
US-listed shares of Japanese automotive companies fell, with Toyota Motor down 4.1 per cent in early afternoon trading and Honda Motor off by 3.8 per cent.
US Treasury Secretary Scott Bessent said earlier on Monday he expected to make several trade announcements in the next 48 hours, adding that his inbox was full of last-ditch offers from countries to clinch a tariff deal before a July 9 deadline.
It was not immediately clear if other letters to leaders would be made public ahead of Wednesday's deadline.
Bessent did not say which countries could get deals and what they might contain.
Trump has kept much of the world guessing on the outcome of months of talks with countries hoping to avoid the hefty tariff hikes he has threatened.
"We've had a lot of people change their tune in terms of negotiations. So my mailbox was full last night with a lot of new offers, a lot of new proposals," Bessent said in an interview with CNBC.
For its part, the European Union still aims to reach a trade deal by July 9 after European Commission President Ursula von der Leyen and Trump had a "good exchange," a Commission spokesperson said.
It was not immediately clear, however, whether there had been a meaningful breakthrough in talks to stave off tariff hikes on the United States' largest trading partner.
Adding to the pressure, Trump threatened to impose a 17 per cent tariff on EU food and agriculture exports, it emerged last week.
The EU has been torn over whether to push for a quick and light trade deal or back its own economic clout in trying to negotiate a better outcome.
It had already dropped hopes for a comprehensive trade agreement before the July deadline.
"We want to reach a deal with the US We want to avoid tariffs," the spokesperson told reporters at a daily briefing.
"We want to achieve win-win outcomes, not lose-lose outcomes."
Without a preliminary agreement, broad US tariffs on most imports would rise from their current 10 per cent to the rates set out by Trump on April 2.
In the EU's case, that would be 20 per cent.
Von der Leyen also held talks with the leaders of Germany, France and Italy at the weekend, Germany said.
Chancellor Friedrich Merz has repeatedly stressed the need for a quick deal to protect industries vulnerable to tariffs ranging from cars to pharmaceuticals.
The German spokesperson said the parties should allow themselves "another 24 or 48 hours to come to a decision".
Germany's Mercedes-Benz said on Monday its second-quarter unit sales of cars and vans had fallen nine per cent, blaming tariffs.