Lee Myung-bak, South Korea's president, is seeking an assurance that the US will not ship beef from cattle older than 30 months, although that is allowed under the existing deal.
But the US has refused to rework the deal that saw South Korea remove restrictions first imposed on its beef imports five years ago.
Chuck Conner, the US deputy agriculture secretary, said there are "national protocols that we have negotiated with the Korean government, and we do not intend to renegotiate those protocols".
In April, Lee struck a deal with George Bush, the US president, to reopen the beef market that was shut down after the first case of mad cow disease was reported in the US in 2003.
South Korea was the third biggest overseas customer for US beef prior to the ban which has long been an irritant in ties between the two countries.
On Wednesday visiting South Korean officials told US legislators that banning older cattle would "go a long way to resolving the issue in Korea".
The Americans agreed that a permanent resolution was critical to getting a free trade agreement between the two countries, and some of them said they were willing to consider changes to the beef deal.
Kevin Brady, a Texas Republican, said the idea of banning older cattle may have merit but he wanted to discuss it with the state's beef producers and sellers.
'Their own problem'
Others, however, took a harder line.
|Kim will push the US for a voluntary ban on|
beef shipments from old cattle [AFP]
Ben Nelson, a Democrat from the cattle-producing state of Nebraska, opposed banning older cattle, saying the move might prompt other countries to demand such concessions.
"They created the problem; they're going to have to find solutions," he said, blaming the South Korean government for successfully stoking fears that US beef was unsafe.
"Now, it's like trying to un-ring the bell or unscramble the eggs, and they've got a serious problem. It does threaten their government.
"Unfortunately, they've made their own nest."
Younger cattle are believed to be less susceptible to mad cow disease, a brain-wasting illness scientists say spreads when cattle is fed with recycled meat and bones from infected animals.
The US banned recycled feeds in 1997.
Both Seoul and Washington say that US beef is safe, citing the Paris-based World Organisation for Animal Health.