Agriculture Minister Chung Woon-chun said earlier Tuesday that Seoul had asked the U.S. to refrain from exporting any beef from cattle 30 months of age and older, considered at greater risk of the illness.
Presidential spokesman Lee Dong-kwan said the president told a weekly Cabinet meeting that "it is natural not to bring in meat from cattle 30 months of age and older as long as the people do not want it."
The spokesman also expressed hope that the United States would respect South Korea's position following large-scale anti-government protests over the weekend.
Alexander Vershbow, the U.S. Ambassador to South Korea, said Washington saw no need to renegotiate an April agreement allowing for American beef imports.
Speaking in Seoul, he said the deal is "based on international science and there is no scientific justification to postpone implementation."
Later in Washington, State Department spokesman Tom Casey said the U.S. looks forward to restarting exports to South Korea "without restrictions."
But he acknowledged that the country has "issues within their system that they need to work through. We respect that."
South Korea agreed in April to reopen its market to U.S. beef after it was blocked for most of the past four and a half years after the first case in the United States of the brain-wasting cattle disease was found in late 2003.
However, after tens of thousands of people rallied over the weekend and a request from the ruling party, the government said Monday it was delaying implementation of the agreement.
The government decided on the delay to "humbly accept the people's will," Chung said.
He said quarantine inspections of U.S. beef will not resume until South Korea receives a response from Washington on the request to avoid exports of older cattle.
South Korea last year briefly allowed imports of boneless beef from cattle under 30 months of age, but they were suspended after bones were found in some shipments from U.S. meatpackers.
The April 18 accord allowed for a resumption of almost all imports - except for cattle parts known as specified risk material such as the brain, skull, eyes and spinal cord - in what had been the third-largest overseas market for U.S. beef. But the much-criticized deal sparked near-daily street protests.
Those demonstrations escalated dramatically over the weekend after the government announced Thursday it would begin inspections of U.S. beef this week, the last step before resuming imports.
Protesters claim U.S. beef is unsafe and say Lee is ignoring their concerns, behaving arrogantly and kowtowing to Washington.
About 8,000 protesters gathered in central Seoul under rainy skies Tuesday night, holding candles - which have become a symbol of the demonstrations - under their umbrellas.
"Lee Myung-bak, step down," they chanted.
Office worker Lee Ye-joo questioned the sincerity of the government's request to the U.S. to avoid exporting older beef.
"It's a show," she said. "I cannot trust the government."
Americans consumed 28.1 billion pounds of beef in 2007, U.S. Department of Agriculture data show, and both U.S. and South Korean officials have repeatedly said American beef poses no safety risk.
On Monday, several leading U.S. beef companies said they would begin labeling shipments to South Korea to indicate the age of the cattle at the time of slaughter.
Vershbow, the U.S. envoy, called that a "very positive step which we hope will provide a way forward in what we recognize is a very difficult situation."
Scientists believe mad cow disease, or bovine spongiform encephalopathy, spreads when farmers feed cattle recycled meat and bones from infected animals. The U.S. banned recycled feeds in 1997.
In humans, eating meat products contaminated with the brain-wasting cattle disease is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady.