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.

President Trump said Japan and South Korea would face tariffs of 25 percent unless they reached an agreement with the United States. Other countries received notice of higher levies.

The duties threatened in the letters effectively match the initial reciprocal tariffs Trump briefly imposed on the countries in April.