– The Associated Press

SEOUL, South Korea – President Obama is opening his pitch for faster work to lock down nuclear material that could be used by terrorists with an up-close look at the nuclear front lines along the heavily militarized border with volatile North Korea.

Obama arrived in Seoul this morning for three days of diplomacy. In the midst of an election year focused on economic concerns at home, he has designed a rare Asia visit that features time in just one country. He’ll use much of the time to keep pressure on North Korea to back off a planned rocket launch and return to disarmament talks.

The goal of the large gathering of world leaders is to secure nuclear material and prevent it from being smuggled to states or groups intent on mass destruction. Progress has been uneven since the ambitious goal of lockdown by 2014 was first set out by Obama at a similar session in Washington in 2010. No breakthroughs are expected now.

Right across the border but not participating: nuclear North Korea, labeled by the White House as “the odd man out.” It is brinksmanship with North Korea and Iran, another nation not invited to the summit, that has dominated much of the nuclear debate and that will cast an unquestionable shadow over talks in Seoul.

Obama has called nuclear terrorism the gravest threat the United States and the world may face. North Korea is a prime suspect in the proliferation of some nuclear know-how, along with missiles that could be used to deliver weapons of mass destruction. Iran is suspected in the arming of terrorists with non-nuclear weaponry, and the United States and other nations suspect Iran’s nuclear energy program could be converted to build a bomb.

Syria, Pakistan and other global trouble spots are also on the agenda for separate meetings with global leaders attending 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 amid confusion and disappointment over the state of diplomacy with the nuclear-armed North.

The symbolic visit to the border separating the Korean peninsula will be the fourth by a U.S. president. Ronald Reagan, Bill Clinton and George W. Bush all visited the DMZ; other U.S. officials regularly go there.

The border zone is a Cold War anachronism, a legacy of the uncertain armistice that ended the Korean War nearly 60 years ago. Hundreds of thousands of troops stand ready on both sides of the border zone, which is littered with land mines and encased in razor wire. Obama officials said the goal is to thank U.S. and South Korean military members and show U.S. resolve from “the front line of democracy” on the peninsula.

The United States has more than 28,000 troops in South Korea.

North Korea plans to launch a long-range rocket next month, which the United States and other powers say would violate a U.N. ban on nuclear and missile activity because the same technology could be used for long-range missiles. Taken by surprise, the United States warned that a deal to resume stalled food aid to the North could be jeopardized if North Korea goes ahead.

The planned missile launch appears part of a long pattern of steps forward, then back in U.S. dealings with North Korea, and plays into Republican claims that Obama is being played for the fool.

Campaign politics surrounding a sitting president typically subside when he is abroad, although Obama’s posture toward threats to America will be scrutinized by his rivals.

The timing comes as daily economic worries, not foreign ones, are driving the concerns of American voters.