SEOUL—President Trump sowed fresh doubts this week about the staying power of his trade deals with a vow to resurrect tariffs on South Korea despite an accord reached last fall.
Trump said Monday that South Korea’s reciprocal tariff rate, as well as levies on autos and other key sectors, would rise back to 25% from their current 15%. He cited delays in South Korea’s legislature approving a pact struck in late October with the country’s president, Lee Jae Myung. Trump didn’t say when the tariff increases would occur.
Seoul officials said Trump’s move took them by surprise. They convened an emergency meeting and diverted South Korea’s industry minister, on a visit to Canada, to Washington, D.C.
Trump’s reversal underscores the asymmetry of his tariff deals: While they exist with him at the executive level in the U.S., in other countries, signoff often must flow through legislatures and regulatory bodies.
The Trump administration has struck more than a dozen trade deals, with the European Union, Japan, the U.K. and various Latin American and Southeast Asian nations. Nearly all aren’t legally binding. Most are a page or two long.
Many deals, including those with the EU and South Korea, haven’t yet won final legislative approval—a move that typically takes many months, if not years. Others, such as with the U.K. and Japan, didn’t need a full parliamentary vote to be completed.
Waiving tariffs on American products, for instance, can require a nation to amend its tax code and establish fresh regulations. The brevity—and breakneck speed—of many of the new trade deals with the U.S. have left ambiguities that vex swift approvals. Political divisions in many nations are adding further delays.
“Trump would like everyone else to be able to snap their fingers and respond with the speed he currently does,” said Deborah Elms, head of trade policy at the Singapore-based Hinrich Foundation. “That is not true in most other countries. That is not a process allowed in most other places.”
The legality of the tools Trump used to impose many tariffs now sits with the Supreme Court, which could provide a ruling next month. That uncertainty has prompted some countries to be patient on landing deals with the U.S. or completing them. The lack of Congressional involvement also means a future American president could reverse course.
The potential reversal of South Korea’s pact follows similar Trump tariff threats aimed recently at Canada and several European nations. The European Parliament last week temporarily froze its approval process for last summer’s EU-U.S. trade deal in response to Trump’s threat of tariffs on countries that opposed his efforts to take control of Greenland.
Trump called off those threats, and lawmakers are set to re-evaluate the situation next week, said Bernd Lange, who chairs the body’s international trade committee.
Big trading partners had offered the Trump administration a mix of trade concessions, state-backed investment and market access. In return, they got a lowering of U.S. tariffs.
That was the arrangement for South Korea, which pledged $350 billion of U.S. investments, plus commitments to buy American energy and other concessions, in return for tariffs dropping to 15% for most of its exports.
A joint fact sheet outlining the accord—which didn’t spell out a timeline for legislative approval—arrived in November. The final steps from there soon became politically contentious.
Lee’s left-leaning ruling party, which controls the 300-seat legislature, argued that ratification was unnecessary because it was a leader-level agreement. The opposition conservatives, citing the large investment figure, demanded legislative approval because the state was assuming a significant financial burden.
The stalemate has lasted for months.
In lieu of ratification, Lee’s Democrats have proposed creating a state body that would oversee South Korea’s investments into the U.S. That bill remains at the committee level.
It doesn’t appear likely that South Korea will back away from the trade deal in protest, though pressure to deliver on a first U.S. investment will rise, said Choi Byung-il, president of a trade strategy unit at the Bae, Kim & Lee law firm in Seoul.
“Trump is tossing the ball to the Korean side: ‘You guys hurry up and do something,’” said Choi, a former South Korean trade negotiator.
Write to Timothy W. Martin at Timothy.Martin@wsj.com