* US imposes steel import quota of 70 pct of 3-yr average

* US exempts tariffs on South Korean steel under quota

* South Korea is 3rd largest steel exporter to US

* S.Korean steelmakers rally, Asian shares steady (Adds context on markets, trade war fears; comment from Hyundai)

By Hyunjoo Jin and Joyce Lee

SEOUL, March 26 (Reuters) - The United States and South Korea agreed to revise a trade pact sharply criticised by U.S. President Donald Trump, Seoul said on Monday, with U.S. automakers winning improved market access and Korean steelmakers hit with quotas but avoiding hefty tariffs.

The planned changes in the U.S.-Korea Free Trade Agreement (KORUS) were seen as limited, leaving South Korea’s key passenger car exports untouched and helping soothe fears that Trump’s tough approach could start a spiralling global trade war.

In April, Trump told Reuters he would either renegotiate or terminate what he called a “horrible” trade deal that has doubled the U.S. goods trade deficit with South Korea since 2012.

Asian shares steadied on Monday, stemming last week’s hefty losses after Trump’s action on steel and aluminium, and his plans to slap tariffs on up to $60 billion in Chinese goods.

The agreement means South Korea will be forced to cut its steel exports to the U.S. by 30 percent of past three years’ average, in exchange for becoming the first U.S. ally to receive an indefinite exemption on steel tariffs imposed by Trump.

“We had heated discussions,” South Korean Trade Minister Kim Hyun-chong said at a media briefing in Seoul. “The latest agreement removed two uncertainties,” he said, referring to steel tariff exemptions and KORUS renegotiation.

Last week, Trump temporarily excluded six trade partners, including Canada, Mexico and the European Union from higher U.S. import duties on steel and aluminium which came into effect on Friday.

South Korea has received a quota of about 2.68 million tonnes of steel exports, or 70 percent of the annual average Korean steel exports to the United States between 2015-2017, which will be exempt from the new tariffs, the ministry said in a statement.

South Korea is not allowed to export steel products exceeding that quota to the U.S. market, a ministry official said.

“This leaves a bad precedent of exchanging steel tariffs - which is a breach of international trade law - for a legitimate free trade agreement, in negotiations,” said Wonmog Choi, professor of law at Ewha Womans University.

TRADE WAR FEARS

South Korea is the third-largest steel exporter to the United States and the world’s top importer of Chinese steel, leading to concerns it was a conduit for China’s excess capacity.

Trump was elected in 2016 after promising to punish what he saw as unfair trade practices by other countries, particularly China.

While Trump was adamant the KORUS deal needed renegotiating, the trade spat risked undermining relations between Seoul and Washington at a crucial time, as Washington and Seoul work closely to try to contain a nuclear-armed North Korea.

“We are at a time when U.S.-South Korea cooperation is needed more than ever ahead of the inter-Korean summit and the summit between North Korea and the United States,” said a senior official at South Korea’s presidential Blue House, who was not authorised to speak to media.

South Korean officials said that while the deal agreed was the best they could hope for, further pressure on trade was likely under Trump’s presidency.

“If President Trump becomes a two-term president ... I believe there will be continuous (trade) risks during that time,” Trade Minister Kim said.

Shares in South Korean steelmakers rallied on Monday, with Dongbu Steel leading gains as tariff exemptions were confirmed.

South Korea’s steel association said in a statement it was a “relief” that South Korea has been excluded from U.S. steel tariffs, but regrettable it was unable to secure higher quotas.

AUTOS CONCESSION

As part of the KORUS revision, the countries agreed to extend U.S. tariffs on Korean pickup trucks by 20 years until 2041.

No South Korean automakers currently export pickup trucks to the United States, but Hyundai Motor said last year it planned to launch a model there to catch up with a shift away from sedans.

Hyundai said on Monday it was “too early to elaborate on the details such as the estimated timing of the model release and production location”. Its shares fell 1.3 percent.

Under KORUS revisions, U.S. automakers will be able to bring into South Korea 50,000 vehicles per automaker per year that meet U.S. safety standards, not necessarily Korean standards, up from 25,000 vehicles previously.

Kim said no automakers previously exceeded the 25,000-vehicle threshold. Ford Motor Co and General Motors each shipped fewer than 10,000 vehicles last year.

“I don’t see a high chance of automakers expanding U.S. imports,” he said.

Kim Jong-hoon, a former chief negotiator for the KORUS FTA, said South Korea had fared relatively well in negotiations.

“South Korea gave concessions in autos in return for steel tariff exemptions,” he told Reuters. “This is not a free trade, but a managed trade.” (Reporting by Joyce Lee and Hyunjoo Jin; Additional reporting by Christine Kim Editing by Lincoln Feast)