Washington, D.C. ? The top U.S. and South Korean trade envoys broke off talks Sunday without resolving a crisis over the resumption of American beef shipments that has shaken South Korea’s pro-U.S. government.

South Korean Trade Minister Kim Jong-hoon, who flew into Washington on Friday for discussions, was returning home, U.S. Trade Representative spokeswoman Gretchen Hamel said.

Kim and Susan Schwab, the U.S. trade envoy, held “frank and candid discussions” Friday and Saturday, Hamel said, but, “in order to find a mutually acceptable solution, both sides need more time to look into technical issues.”

She said officials from the countries will stay in contact.

The talks, which focused on the importation of U.S. beef from cattle below 30 months of age, came as thousands of people protested in Seoul, demanding that a beef import deal settled in April be renegotiated and urging South Korean President Lee Myung-bak to resign.

Lee has vowed not to allow the import of beef from cattle older than 30 months. Scientists think infection levels of mad cow disease increase with age.