"No major progress has been made," a South Korean negotiator told The Associated Press, asking not to be named, citing the sensitivity of the issue.
South Korea explained its risk assessment of the U.S. beef to American officials, the official said without giving further details. The talks were set to resume Friday.
The two days of meetings in Seoul came a week after South Korea suspended all American beef imports over mad cow fears after inspectors found a recent shipment contained bone that is banned.
South Korea agreed last year to import only boneless U.S. meat from cattle less than 30 months old - believed to be safer from the disease - partially lifting an almost three-year ban imposed on American beef after the brain-wasting disease was discovered in the U.S.
This week's talks are aimed at revising the import conditions. South Korean officials said they would maintain the current import ban until after a new set of conditions are worked out.
The United States wants Seoul to fully open its beef market, citing a ruling by the World Organization for Animal Health earlier this year that the United States was a "controlled risk nation," a category that means countries can export beef irrespective of the animal's age.
Washington seized on the announcement as proof that U.S. beef is safe.
Representing Washington in this week's talks is Chuck Lambert, deputy under secretary for marketing and regulatory Programs at the Agriculture Department. His South Korean counterpart is Lee Sang-kil, director of the Agriculture Ministry's Livestock Bureau.
South Korean officials declined to comment on the prospect of this week's negotiations.
But Yonhap news agency said Seoul is expected to agree to import rib bones while maintaining a ban on imports of "special risk material" such as spinal columns, skulls and eyes, as well as intestinal parts - believed at risk of carrying the brain-wasting disease.
Rib meat is used to make galbi, a popular dish of marinated meat barbecued at diners' tables that is a mainstay of the Korean diet.
South Korea was the third-largest foreign market for American beef before it banned U.S. imports.