President Donald Trump is threatening to impose higher tariffs on Japanese exports to the United States over what he claims is the country’s unwillingness to buy American-grown rice.

Japan will not sacrifice the agricultural sector as part of its tariff talks with the United States, its top negotiator said on Tuesday, after President Donald Trump complained that the key Asian ally was not buying American rice.

We’ll just be sending them a letter,” Trump said, after suggesting Japan wouldn’t agree to boost U.S. rice imports.