This year's talks are affected by budget-cutting efforts in Washington and South Korea's increasingly tense relationship with the North, Yonhap News Agency reported.
The United States currently has 28,500 soldiers in South Korea. South Korea began paying for part of the effort in 1991, with its share currently at 40 percent.
U.S. officials hope to negotiate an increase in the South Korean share to 50 percent.
The talks in Seoul are expected to conclude Thursday.