President Trump said the U.S. will impose steep levies on imports from 14 countries starting next month, heightening pressure on trading partners.

The country’s new president, in office for a little over a month, had just dispatched his senior deputies to Washington to try to work out a trade deal.

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.