President Donald Trump is threatening to slap 25 percent tariffs on Japan and South Korea as it continues to negotiate trade deals with the countries ahead of a new Aug. 1 deadline.

The rates, which were detailed in letters Trump posted to Truth Social Monday afternoon, effectively match the initial reciprocal tariffs Trump briefly imposed on the two countries in April. But the letters extend the deadline for the tariffs to kick in — from July 9 to Aug. 1 — allowing negotiations to continue, as Trump continues to press the countries to increase their defense spending and boost agricultural imports, among other measures.

“We invite you to participate in the extraordinary Economy of the United States, the Number One Market in the World, by far,” Trump wrote in the letters.

The letters also contained a warning: If either country attempts to impose retaliatory tariffs, Trump will add the equivalent rate to the 25 percent tariff facing the countries’ exports.

In the letters, Trump said that he will “perhaps, consider an adjustment to this letter” if the countries open their markets to more U.S. goods and reduce tariff and non-tariff barriers.

Still, the markets started down on Monday and the Dow, S&P 500 and Nasdaq all dropped slightly as Trump’s letters went public on Truth Social — a signal that the market could once again be bracing for a trade war, rather than a series of deals with the country’s major trading partners. The president initially set his “reciprocal” tariff rates to go into effect on April 9, but delayed the date 90 days after financial markets and consumer confidence plummeted.

Trump said the tariffs on Japan and South Korea would be separate from any “sectoral” tariffs that he imposes. That appears to refer to the duties that he has already imposed on autos, auto parts, steel and aluminum under Section 232 of the 1962 Trade Expansion Act, which gives the president broad authority to restrict imports to protect national security.

Both Japan and South Korea are large auto and steel exporters to the United States, and have been looking for relief from those duties. In addition, Trump is threatening to impose additional Section 232 tariffs on seven other sectors, including pharmaceuticals, semiconductors, copper, aircraft and jet engines, lumber, trucks and critical minerals.

Trump’s top trade officials have for months attempted to secure a deal with both Japan and South Korea — both of whom are key strategic partners in the Pacific as the U.S. attempts to counter China’s rise. But a presidential election in South Korea stalled substantive talks for months and, with a looming Upper House election this month, Japan insisted that any deal would have to fully remove auto tariffs.

Spokespeople for the Japanese and South Korean embassies did not immediately respond to a request for comment.

Asked about the letters, one Asian diplomat who works with negotiators in both Japan and South Korea, and was granted anonymity to candidly discuss the state of talks, said the president could change his mind again. “It changes, what he says, by the day — sometimes by the week.”

The letters also offer no certainty for countries looking for assurances that any deals they negotiate will be lasting. In the letters, Trump said his tariffs “may be modified, upward or downward, depending on our relationship with your Country.”

Trump said he was imposing the duties to help reduce the “very persistent” trade deficits with the two countries — meaning they export more goods to the U.S. than they buy from the U.S. — which the president blamed on Japan and South Korea’s tariffs and other trade barriers.

However, most economists disagree with that analysis, saying that macroeconomic factors like relative savings rates play more of a role in driving the overall U.S. trade deficit.

The bilateral U.S. trade deficits with Japan and South Korea last year were $68.5 billion and $66 billion, respectively, the seventh and eighth largest the United States had with any country.

Ari Hawkins and Doug Palmer contributed to this story.