President Trump posted his letter to South Korea’s newly elected president, Lee Jae Myung, on social media just days after Mr. Lee had sent senior aides to Washington to accelerate efforts to work out a trade deal.

Mr. Trump threatened to impose a 25 percent tariff on South Korean exports to the United States starting Aug. 1 unless the longtime American ally lowered what he believes are unfair trade barriers.

Mr. Lee, who took office a little more than a month ago, had grappled with the negotiations over the tariffs, which threatened to complicate his country’s seven-decade alliance with Washington. The talks had stalled after South Korea endured a prolonged political crisis triggered by former President Yoon Suk Yeol’s short-lived imposition of martial law in December and his subsequent impeachment.

Hours after Mr. Trump posted his letter on Truth Social, senior government policymakers called a meeting in Seoul where Mr. Lee’s chief policy coordinator, Kim Yong-beom, said that protecting South Korea’s national interest was more important than reaching a quick deal, according to Mr. Lee’s office.

South Korean officials sounded relieved that they would have the rest of the month to try to work out a compromise with the Trump administration before the tariffs would take effect.

“We will double our efforts to produce a result mutually beneficial for both sides,” the South Korean trade ministry said in a statement on Tuesday. “We will improve domestic institutions and regulations to help ease the United States’ concern about its trade deficit.”