President Donald Trump said Monday that he was increasing tariffs on imported autos, pharmaceuticals, and lumber from South Korea from 15% to 25% because of a delay in that country's legislature approving a trade deal with the United States reached last summer.

Hyundai Motor shares fell as much as 4.77% on the heels of the news, but later pared losses and were last down 0.81% in volatile trading. The South Korean-based is the largest importer of new vehicles from that country into the United States.

The share price of Hyundai's subsidiary Kia dropped nearly 3.5%, and the affiliated Hyundai Mobis was down 5%, Reuters reported.

"South Korea's Legislature is not living up to its Deal with the United States," Trump said in a Truth Social post.

"President Lee [Jae Myung] and I reached a Great Deal for both Countries on July 30, 2025, and we reaffirmed these terms while I was in Korea on October 29, 2025. Why hasn't the Korean Legislature approved it?" Trump asked.

"Because the Korean Legislature hasn't enacted our Historic Trade Agreement, which is their prerogative, I am hereby increasing South Korean TARIFFS on Autos, Lumber, Pharma, and all other Reciprocal TARIFFS, from 15% to 25%," he wrote.

CNBC has requested comment from South Korea's embassy in Washington, D.C., on Trump's announcement.

Reuters reported that South Korea's presidential Blue House said that the U.S government had not officially notified South Korea of the tariff hike.

The news outlet also reported that South Korea's presidential adviser would hold a meeting with related ministries to discuss measures related to the increase.

South Korea is one of the biggest exporters of goods to the United States.

The U.S. imported $131.6 billion in goods from South Korea in 2024, according to the Office of the U.S. Trade Representative.

In the trade deal announced in July, Trump said the U.S. would impose blanket tariffs of 15% on imports from South Korea, 10 percentage points lower than the level he had threatened earlier that same month.

Trump also said at the same time that South Korea had agreed to "give to the United States $350 Billion Dollars for Investments owned and controlled by the United States, and selected by myself, as President."

The U.S. Supreme Court in November heard arguments in a case challenging the legality of Trump's unilaterally imposed tariffs, with justices expressing skepticism that he had the power to levy such duties without authorization of Congress.

The Supreme Court has yet to issue its decision in that case.