The technical-level negotiations began Thursday and may go on into Sunday, Yonhap reported.
The pact was signed in 2007 but failed to win parliamentary approval in either country as the U.S. Congress demanded South Korea further open its auto and beef markets.
The countries hope to complete the deal before their presidents meet at the Group of 20 summit, which begins Thursday in Seoul.
The chief remaining issues involve U.S. auto industry complaints about South Korean tax and regulatory policies they say keep their cars out of the market, and remaining restrictions on imports of U.S. beef.
Washington wants South Korea to relax its fuel mileage and emissions standards and drop a provision that allows refunds for tariffs on imported auto parts.
South Korea imports beef only from cattle younger than 30 months because of mad cow disease. The United States reported three cases of the disease between 2003 and 2006. Seoul banned imports of American beef in 2003 before resuming them in 2008.