S. Korea will also consider the United States standards when it revises its standards.
Trump's proposed tariffs set off jostling among USA allies and trading partners including Australia, the European Union and Japan, as well as South Korea, to win exemptions.
South Korean Trade Minister Kim Hyun-chong announced Monday that the U.S. and South Korea have agreed to amend the six-year-old Korea-U.S. At present, vehicles manufactured in the United States and exported to South Korea include Mercedes-Benz GLE, GLE Coupe and GLS and Toyota Sienna. But South Korea's steel tariff exemption is subject to a quota of about 2.7 million tons of steel products a year, about 74 percent of its exports in 2017.
The US secured a 20-year delay in eliminating tariffs on South Korean pickup trucks.
The US trade deficit with South Korea has declined since 2014, to $10.3 billion a year ago.
"If the result is too lopsided in favor of one side, there might have to be a renegotiation later", Trade Minister Kim Hyun-chong said Monday in Seoul, adding that he hoped there would be no need for further renegotiations. "We also removed two uncertainties by getting tariff exemption for our steel and striking FTA deal".
Since its introduction in 2007, KORUS has had more than its fair share of critics. Despite the fact that South Korea's trade surplus with the United States has declined slightly over the past 12 months, it still stands at around $18bn, with the majority stemming from the automobile sector.
The resolution of these trade disputes, which have created tension in an important bilateral relationship at a crucial time in dealings with North Korea, allow the US and South Korea to face North Korea from a more stable, unified position.
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In Washington, rallygoers also saw the West family - Kanye, Kim and North - and the Clooneys - Amal and George - among the crowd. There were also rallies in Brisbane, Australia; Copenhagen, Denmark; Madrid; Paris and Rome.
Last week, Trump temporarily excluded six trade partners, including Canada and Mexico, Australia and the European Union from higher USA import duties on steel and aluminium which came into effect on Friday. Just last week U.S. officials had said Friday an agreement could be announced in the coming week.
South Korea and the United States earlier this month held a third round of negotiations on revising their trade pact.
American auto brands accounted for 6.8 percent of all imported cars sold in South Korea as of February, falling far behind European brands, with 77.8 percent, and Japanese, with 15.8 percent, according to the Korea Automobile Importers and Distributors Association.
She said: "Recently we have seen how [trade] is used as a weapon to threaten and intimidate us".
Previously lifted tariffs on South Korean auto exports won't be reimposed, Kim said, and the nation planned to ease some of the safety and environmental regulations on imported United States cars.
He described the negotiations as "fierce" but insisted, "As a negotiator, I can say it was a negotiating table where I had nothing to feel inferior about".
Kim told Reuters no automakers previously exceeded the 25,000 vehicle threshold.
Cecilia Malmstrom, European Union commissioner for trade, also said the bloc will "stand up to bullies" that create trade wars, at a conference in Brussels.
Washington also exempted Seoul from additional 25% tariffs on steel which the USA imposed to most of nations, except Canada, Mexico, the European Union, Australia, Argentina and Brazil.