President Trump has threatened to hit more than a dozen nations with steep import duties by next month. Mr. Trump initially posted two nearly identical letters on Monday, addressed to Japanese Prime Minister Shigeru Ishiba and South Korean President Lee Jae-myung,

In letters so far to 14 countries, including smaller exporters to the United States such as Serbia, Thailand and Tunisia, Trump hinted at opportunities for additional negotiations, even while warning that reprisals would draw a like-for-like response.