Protests against the resumption of beef imports have escalated
South Korea says it has requested a key change to a US beef import deal, after weeks of anti-government protests.
Farming Minister Chung Woon-chun said Seoul had asked the US to stop exports from cattle aged more than 30 months, in an effort to ease health fears.
Imports would be delayed until the US responded, he said. The US said the two nations were discussing the issue.
At the weekend, tens of thousands held protests over renewing US beef imports, citing fears of mad cow disease.
More than 200 people were arrested and dozens were hurt as police and demonstrators clashed in the capital, Seoul.
White House spokesman Tony Fratto said on Tuesday: "We're in communication with the [South] Korean government to try to understand their plan.
"We'll continue to work with US industry and our Korean counterparts to ensure that our concerns are addressed."
The beef issue has become a major challenge to new President Lee Myung-bak, with support for him plummeting.
South Korea suspended most US beef imports after concerns about BSE
South Korea used to be a major market for US beef, but banned most imports in 2003 after an outbreak of mad cow disease, or BSE, there.
In April Mr Lee's government agreed to relax most of the restrictions - after US lawmakers made it clear they would not back a bilateral trade deal without concessions on beef.
The move triggered public anger, even though the government has described concerns over the beef as "unfounded rumours".
Vigils and rallies have taken place on an almost daily basis since the agreement was announced.
In recent days they have grown in size and there have been scuffles and clashes. Some of the demonstrators have called on Mr Lee to step down.
Late on Monday, the government announced that it was delaying implementation of the beef deal.
Then on Tuesday, Mr Chung said that the government was looking to renegotiate it.
"In response to public safety concerns over US beef, the government had asked the United States to stop exports of beef from cattle older than 30 months old," he told journalists.