Washington DC - President Donald Trump said Monday he was slapping 25% tariffs on Japan and South Korea, in his first letters to trading partners ahead of a deadline to reach a deal with Washington.

Trump had said over the weekend that starting from Monday, he would send a first batch of up to 15 letters to countries informing them that he would reimpose harsh levies that he had postponed in April.

In near-identically worded letters to the Japanese and South Korean leaders, Trump said the tariffs would apply from August 1 because their trading relationships with Washington were "unfortunately, far from Reciprocal."

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Trump warned the countries, both key US allies in East Asia, of an escalation if they responded to the new US tariffs.

But he also said he was ready to modify levies "downwards" if Japan and South Korea changed their trade policies.

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

Trump originally announced sweeping tariffs on world economies on what he called "Liberation Day" on April 2, claiming the US was being "ripped off."

Amid market turmoil, Trump then suspended the initial tariffs for 90 days, a deadline that expires on Wednesday.

But the Trump administration has said that the duties will not "boomerang" back until August 1, apparently extending the deadline despite denials from officials.

While the Trump administration has signaled hopes of striking dozens of deals by early July – at one point boasting of "90 deals in 90 days" – there have been limited results so far.

Washington has unveiled pacts with only Britain and Vietnam, while the US and China agreed to temporarily lower tariff levels on each other's products that earlier reached three-digits.