SEOUL, South Korea - President Bush pledged yesterday to come up with measures to ensure that beef from older cattle - considered at greater risk of mad cow disease - is not exported to South Korea, Seoul's presidential office said.

Bush made the remark during a phone conversation with South Korean President Lee Myung-bak, the presidential Blue House said. Lee's fledgling government has been battered by daily protests over an April agreement to resume imports of U.S. beef.

"President Bush said he sufficiently understands South Koreans' concerns and worries," it said in a statement. "In this regard, [Bush] pledged to prepare specific measures to make sure that beef from cattle aged 30 months or older is not exported to South Korea."

It did not say what those measures would be.

The White House did not respond to requests for comment on the South Korean description of the Bush-Lee conversation.

In Washington earlier, White House spokesman Gordon Johndroe said Bush assured Lee the U.S. government "is cooperating closely with the South Korean government and ready to support American cattle exporters as they reach a mutually acceptable solution with Korean importers on the beef trade."

Lee's call to Bush underscored the new South Korean leader's political dilemma - caught between a pledge to his country's most important ally and the anger of South Korean citizens who have taken to the streets by the tens of thousands.

It was unclear whether Bush's promise announced by South Korea would be enough to ease the opposition to imports of U.S. beef.

Even as the two presidents spoke, a crowd estimated by police at 40,000 rallied in central Seoul against the beef import deal, which they say fails to protect the country from mad cow disease by allowing meat from cattle of any age.

Clashes with police occurred downtown as sporadic protests spilled over into the early hours today.

U.S. beef has been banned from South Korea for most of the last 41/2 years since the first case of mad cow disease in the United States was discovered in late 2003. Two subsequent cases have been found.