The United States and South Korea have successfully concluded a free trade agreement, Steve Norton, a spokesman for the Office of the US Trade Representative, said Monday, wrapping up almost 10 months of contentious negotiations. The deal, which requires legislative approval in both countries, is the biggest for the United States since the North American Free Trade Agreement signed in 1992 and ratified in 1993. It is the biggest ever for South Korea. The governments launched negotiations last June in Washington, alternating meeting sites between the two countries. Both sides strongly advocated a deal, saying it would boost trade and economic growth in the two economies. Differences over trade in automobiles, agriculture, textiles and other issues, including the staus of South Korean goods manufactured at a small enclave in North Korea, had thrown up obstacles.