Steel products are piled at the Port of Pyeongtaek in Gyeonggi Province, Monday. Yonhap

U.S. President Donald Trump alarmed Korean steelmakers, Monday, with his declaration of additional 25 percent tariffs on all steel and aluminum imports into his country.

Amid the growing uncertainty about the Korean exports of steel plates and pipes to the United States, investors dumped their shares in POSCO, Hyundai Steel and Dongkuk Steel at the beginning of Monday's trading session, following the news reports on Trump's remarks.

On the same day, the government also convened meetings to discuss possible countermeasures against the U.S. tariff plan.

Trump told reporters on Air Force One, Sunday (local time), that he would announce the new metal tariffs a day later. This is the first time for his second administration to mention specific sectors that will be subject to universal tariffs.

"Tariffs are going to make it very successful again, and I think it has good management," Trump said of U.S. Steel, addressing his government's decision to allow Japan's Nippon Steel to invest in the American steelmaker on condition of not becoming its largest shareholder.

However, the U.S. president did not tell the press that day whether there would be any changes to tariff-free import quotas.

Then-U.S. President Donald Trump holds up a proclamation on steel imports during an event at the White House in Washington in this March 2018 photo. AP-Yonhap

During Trump's first term, Korea accepted up to 2.63 million tons of tariff-free steel export quota, as he decided in 2018 to impose 25 percent tariffs on all steel imports and 10 percent tariffs on all aluminum imports in accordance with Section 232 of the Trade Expansion Act of 1962.

The amount was 70 percent of the annual average of Korean steel exports to the U.S. between 2015 and 2017.

Since then, Korean steelmakers have had to negotiate with each other every year to determine their respective shares in the 2.63 million tons, before they export their products to the U.S.

"Considering Trump's previous threats to impose tariffs on Canada and Mexico in spite of their trilateral free trade pact, he is expected to apply zero-base principles to import quotas," an official from one of Korea's major steelmakers said on condition of anonymity. "His government is likely to ask us to contribute more the U.S. economy, in exchange for maintaining the tariff-free quota."

In an email sent to the press earlier this month, the White House mentioned Hyundai Steel's "active" consideration of building a U.S. plant as one of the reasons that Trump's tariffs are a "necessary solution."

The steelmaking unit of Hyundai Motor Group, however, reiterated last Friday that nothing has been decided yet regarding this issue.

"We are considering various measures to secure sustainable growth and global competitiveness," Hyundai Steel said in its regulatory filing.

Awaiting further details about the U.S. tariffs on steel and aluminum, domestic steel industry officials urged the government to prevent Washington from making "unreasonable" demands.

They advised the government to emphasize the possibility of U.S. inflation as a result of the steel tariffs, explaining the importance of Korean steel products to the U.S. energy supply chain.

The finance ministry said Monday that acting President Choi Sang-mok talked with foreign and trade ministers about how to respond to Trump's remarks on the 25 percent tariffs on steel and aluminum.

The trade ministry also convened an emergency meeting at the Korea Iron & Steel Association.

"The government will mobilize all possible networks, including diplomatic missions in the U.S., to find out about more specifics," Deputy Trade Minister Park Jong-won said. "In order to minimize impacts on Korean companies, the government will cooperate closely with the industry."