By Jack Kim

SEOUL (Reuters) - The United States and South Korea agreed the biggest U.S. trade pact for 15 years on Monday with only minutes to go before a deadline.

In one major surprise, South Korea said the United States agreed to give, at least in principle, preferential treatment to South Korean products made in North Korea.

The deal to cut tariffs and remove trade barriers follows nine months of talks, and sometimes violent protests in South Korea, mostly over fears that heavily subsidised farmers could not survive a flood of cheaper U.S. farm products.

"We expect the ... (agreement) would provide a stepping stone that would catapult Korea into an advanced economy," a press official quoted President Roh Moo-Hyun as saying.

U.S. negotiators were equally enthusiastic.

"In the agricultural sector, you're going to see substantial new market access for America's agricultural producers in a fast growing, wealthy market ... it's a great deal for America's farmers and ranchers as well," Deputy U.S. Trade Representative Karan Bhatia told a small group of reporters including Reuters'.

Seoul agreed in the end to phase out its 40 percent tariff on U.S. beef over 15 years, but did not budge on the most sensitive, and heavily protected, farm product -- rice, something Bhatia said he wished had been included.

On another major sticking point, the two agreed to open their markets more to each other's autos.