President Donald Trump on Monday set a 25% tax on goods imported from Japan and South Korea, as well as new tariff rates on a dozen other countries.

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.

Japan and South Korea, the United States’ closest security allies in Asia and key partners in its effort to counter China, were sent scrambling Tuesday after the U.S. president renewed his threat to impose steep tariffs on their goods,