(Refiles April 25 story to change second reference to South Korean president's name in paragraph 11) (Adds business group letter, details in paragraphs 4-6)

WASHINGTON, April 25 (Reuters) - The U.S. Senate's top lawmaker on international trade issued a stern missive this week to South Korea's president, warning that failure to end a row over beef imports will doom a newly minted trade pact.

"My support for (U.S.-Korea trade) agreement hinges on whether Korea commits to lift its unscientific ban on exports of American beef," Sen. Max Baucus, the Montana Democrat who chairs the Senate Finance Committee, wrote in a letter to President Roh Moo-hyun.

In the letter, which was released on Wednesday, Baucus threatened to sink the bill in his committee, which oversees trade, unless "American beef is flowing into the Korean market," including bone-in cuts from animals of any age.

Meanwhile, a broad-based coalition of U.S. business groups wrote Baucus and other members of Congress to urge approval of the pact, which also faces strong opposition from organized labor and U.S. carmakers.

"The U.S.-Korea FTA stands to bring real benefits for U.S. workers and businesses," the group said in a letter days after a senior Democrat in the House of Representatives criticized the pact.

House Ways and Means Committee Chairman Charles Rangel, a New York Democrat, said there were three main problems with the agreement -- "automobiles, automobiles, automobiles" -- in reference to provisions that would open the U.S. market to more South Korean cars.

Politicians and industry officials have been hopping mad since South Korea rejected a series of three beef shipments that contained trace bone chips.

Since late last year, beef trade has been stalled, stymieing exporters' hopes of a swift return to the booming exports they enjoyed before 2003, when South Korea closed its market to U.S. beef based on fears of mad cow disease.

Until then, South Korea had been the third-biggest customer for U.S. beef.

Even the Bush administration, which negotiated the deal, acknowledges that the standoff imperils its approval. If passed, the pact is expected to add up to $20 billion a year to the two countries' trade.

Baucus told Roh he wanted to see a change in import policy following a ruling in May from the World Organization for Animal Health (OIE), which is expected to formally classify the United States as a "controlled risk" country.

"Your time frame for resolving this issue is critical," the senator said.

U.S. lawmakers are making a similar case with Japan, which has so far refused to expand import rules that currently allow only U.S. meat from animals 20 months old or younger.

The South Korea pact also faces opposition from some U.S. lawmakers, who are unsatisfied by its provisions to open South Korea's auto market to U.S. cars.

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