Donald Trump's insistence that "spoiled" Japan imports more US rice is adding to Prime Minister Shigeru Ishiba's problems ahead of elections that could sink his premiership after less than a year in office.

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.