On world stage, Obama confronts nuclear threats

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President Barack Obama waves to media as he walks from the White House to board Marine One, Friday, March 23, 2012, in Washington, as he travels to Seoul, Republic of Korea. (AP Photo/Carolyn Kaster)

SEOUL, South Korea (AP) — President Barack Obama has begun a visit to South Korea that aims to mount pressure on an isolated North Korea and intensify international efforts to keep nuclear material out of the hands of terrorists.

Obama arrived in Seoul on Sunday morning, local time, for three days of diplomacy. In the midst of an election year focused on economic concerns at home, Obama has designed a rare Asia visit that features time in just one country.

At its core is a progress-check summit of more than 50 nations on Obama's goal of locking down nuclear material around the world by 2014.

Obama's first business: a visit to the volatile Demilitarized Zone separating North and South Korea, a show of strength by the U.S. as North Korea defiantly plans a new rocket launch.