Razor-wire close to the border, President Barack Obama on Sunday paid his first visit to the tense zone separating North and South Korea amid new nuclear tensions. He told American troops stationed nearby they are protectors of "freedom's frontier."

Obama shook hands and spoke briefly in the dining hall at a U.S. military camp just outside the 2.5-mile-wide Demilitarized Zone, then walked into the heavily patrolled no-man's land to tour a small post where South Korean forces patrol just 100 yards from the demarcation line.

The president, positioned behind bulletproof glass, peered through binoculars across the line that has bisected the Korean peninsula for 60 years. He spent about 10 minutes at the observation post, looking first toward North Korea, then back to the South.

It was an unmistakable show of force to communist North Korea and its new leader at a time of diplomatic standoff. Obama underscored the Cold War symbolism by making the tour his first order of business ahead of a gathering of world leaders pledged to keep nuclear materials safe. Nuclear-armed North Korea will not attend.

The U.S. is threatening to cancel planned food aid to the North over its announcement that it will launch a long-range rocket next month, news that overshadows the gathering of world leaders committed to nuclear security that Obama will attend in Seoul. Obama was holding bilateral talks with South Korean President Lee Myung-bak later in the day, followed by a news conference.

"I could not be prouder of what you're doing," Obama told smiling American troops at Camp Bonifas at the edge of the DMZ. Obama said the same is true at every U.S. military post, but "there's something about this spot in particular."

"You guys are ... at freedom's frontier. When you think about the transformation that has taken place in South Korea during my lifetime, it is directly attributable to this long line of soldiers, sailors, airmen, Marines, Coast Guardsmen who were willing to create the space and the opportunity for freedom and prosperity," Obama said.

Obama's visit takes place as North Koreans mark the end of the 100-day mourning period for longtime leader Kim Jong Il, who died of a heart attack in December. Since Kim's death, son Kim Jong Un has been paying a series of high-profile visits to military units and made his own trip to the "peace village" of Panmunjom inside the DMZ earlier this month.

"The contrast between South Korea and North Korea could not be clearer, could not be starker," Obama told the troops.

That was a reference to the political freedom and prosperity in democratic South Korea, and the repression and desperate food shortages of the North.

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.

Obama also planned to hold separate meetings on the sidelines of the nuclear summit with several world leaders. After returning to Seoul from the DMZ, Obama huddled with Turkish Prime Minister Tayyip Erdogan for talks heavily focused on the ongoing violence in Syria.

Obama said the two leaders were committed to bringing about change in Syria, where the U.N. estimates 8,000 people have been killed in clashes between government forces and opposition groups. Turkey, which shares a border with Syria, will host an upcoming meeting of the diplomatic "Friends of Syria" group, a gathering Obama said would focus on humanitarian aid and political change.

Erdogan said "we cannot remain a spectator to these developments."

Much of Obama's time was to be spent keeping up pressure on North Korea to back off the planned rocket launch and return to disarmament talks.

The South Korean and U.S. militaries have found that North Korea has moved a long-range rocket to its northwestern launch site in preparation for its launch, South Korean Defense Ministry and Joint Chiefs of Staff officials said Sunday. The officials _ speaking on condition of anonymity citing department rules _ refused to provide further details.

Obama's visit to the border separating the Korean peninsula is 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 satellite using a long-range rocket next month, which the U.S. 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 U.S. warned that a deal to resume stalled food aid to the North could be jeopardized if North Korea goes ahead.

The U.S. considers the rocket launch practice for a ballistic missile test and a violation of North Korea's international responsibilities. The planned launch is yet another setback for the United States in years of on-again, off-again attempts to launch real negotiations. The announcement also played into Republican criticism that Obama had been too quick to jump at a new chance for talks with the North Koreans.

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. Yet the setting does give Obama a few days to hold forth on the world stage while, back home, Republican presidential candidates keep battling each other.

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 2010, when Obama set an ambitious goal of locking down vulnerable nuclear materials by 2014. No breakthroughs are expected now.

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 U.S. and other nations suspect Iran's nuclear energy program could be converted to build a bomb.

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Feller is AP's White House correspondent; Gearan is an AP national security writer. Gearan reported from Seoul, South Korea.

AP Writers Jean H. Lee and Hyung-jin Kim contributed to this report.