(Adds details of deal, U.S. senator quote, paragraphs 8, 19-20)

By Jon Herskovitz

SEOUL, June 21 (Reuters) - South Korea, which reworked an unpopular U.S. beef import deal that sparked mass street protests, could soon resume imports once a legal step has been completed, the trade minister said on Saturday.

South Korea and the United States reached a private-sector deal in Washington this week to limit U.S. beef exports to cattle under 30 months old, which are thought to pose less risk of mad cow disease to consumers, Trade Minister Kim Jong-hoon told reporters on Saturday.

The Bush administration on Saturday confirmed that U.S. beef exporters are set to resume shipments, under restrictions.

"Korean beef importers and U.S. exporters have reached a commercial understanding that only U.S. beef from cattle under 30 months of age will be shipped to Korea, as a transitional measure, to improve Korean consumer confidence in U.S. beef," U.S. Trade Representative Susan Schwab said in a statement issued in Washington.

Kim said after the news conference the publication of the notice that would start the legal process to resume quarantine inspections was expected "some time next week."

Once the legal notice is published, U.S. beef that has been in frozen storage in South Korea for months could be inspected and then head to store shelves.

This could happen as early as next week, although Kim later said that it might not be so soon because of the sensitivity of the issue for South Koreans.

The arrangement, which officials said would take effect shortly, will also allow Seoul to block exports of any specific product or from any processing plant if it finds "serious" violation of trade rules.

South Korea and the United States first reached a deal in April that would allow in all cuts of U.S. beef regardless of age.

But this quickly led to a crisis for the government of new President Lee Myung-bak, who sent Kim to Washington this week to add safeguards to the original agreement.

The April beef deal was meant to help a separate bilateral free-trade accord that U.S. congressional leaders threatened to block unless South Korea opened up its market to beef imports.

But widespread concern over mad-cow disease in U.S. beef quickly turned the issue into a lightning rod for a broad range of grievances that sparked protests against Lee's government, increasingly seen as out of touch with the public.

Lee apologised on Thursday for the April beef deal and said he will sack top aides but analysts said that might not be enough to reverse a slide in his public support rate, which has plummetted to under 20 percent.

About 10,000 people were expected to hold a protest in central Seoul later on Saturday, police said.

Kim said it was now up to South Korean consumers to decide if they wanted the U.S. product that experts said would sell for about half the price of similar cuts of domestic meat.

"South Korea can send back shipments that do not have U.S. Department of Agriculture verification... and this (verification) measure will be in place indefinitely unless consumer confidence is restored," the Trade Ministry said in a statement.

Lee, a former Hyundai construction CEO, in December won a sweeping victory in December's presidential race with pledges to pass pro-business reforms to revitalise the economy.

But Lee's plans for corporate tax cuts, privatisation of state firms and mortgage-debt relief for low-income households are expected to remain on hold unless he can win back support, analysts said.

In Washington, Sen. Max Baucus who heads the powerful Senate Finance Committee objected to the new arrangement, which he said "effectively changes the April 18 accord struck between Korea and the United States."

"The implications of this agreement set an unfortunate precedent for U.S. beef trade with Korea and other countries," the Montana Democrat said in a statement. (Additional reporting by Lee Jin-joo and Miyoung Kim, and by Missy Ryan in Washington, Editing by David Fogarty and Vicki Allen)

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