Both countries will be subject to a 25 percent tariff on August 1, Trump said in emails posted on Truth's social media account, potentially giving the countries more time to negotiate deals.

In two nearly identical letters, Trump said he particularly disliked the United States' trade deficit with both countries, meaning America buys more goods from them than American companies export to those countries. Trump also said the tariffs would be in response to other policies he said were hindering American goods from being sold abroad.

"Please understand that the 25 percent figure is far less than what is needed to close the trade deficit discrepancy we have with your country," Trump said in both letters.

He urged both countries to manufacture goods in the United States to avoid tariffs.

“There will be no tariffs” if Korea or Japan or “your country’s companies decide to build or manufacture products in the United States, and, in fact, we will do everything possible to get approvals quickly, professionally and routinely — in other words, within a few weeks,” he added.