South Korean negotiators say demands for U.S. access to the South Korean rice market could wreck a major trade deal.
South Korea's top chief negotiator Kim Jong-hoon, spoke with reporters as negotiators started a final round of talks in Seoul.
The United States wants Seoul to allow more rice imports.
U.S. officials intend to finish negotiations by the end of March in order to get congressional approval of the deal before President Bush's authority to "fast track" obtain quick approval of trade pacts expires on July 1.
South Korea has been pushing for the pact despite protests from farmers and activists who say the agreement will threaten their livelihood.
On Sunday, thousands of activists marched toward the U.S. Embassy in Seoul, urging the government to scrap the deal.
If reached, the deal would be the largest U.S. trade agreement since the 1994 North American Free Trade Agreement. It must also be approved by the South Korean parliament.
Some information for this report was provided by AFP, AP and Reuters.