Obama visits tense Korean border ahead of nuclear summit US President Barack Obama looks towards North Korea from Observation Post Ouellette during a visit to the Joint Security Area of the demilitarised zone Photograph: Jewel Samad/AFP/Getty Images President Obama has told US troops they stand on "freedom's frontier" during a visit to South Korea's heavily fortified border with North Korea on the eve of an international summit that will address mounting concerns over the North's nuclear weapons programme. Wearing a black bomber jacket, Obama told US troops at Camp Bonifas that they stood between a free and prosperous South Korea and repression in the North. "I could not be prouder of what you do," he said. The same was true of US military bases worldwide, he added, "but there's something about this spot in particular." Obama's visit to the Demilitarised Zone, a 2.5-mile-wide stretch of land that has separated the two countries since the end of the Korean War in 1953, came as North Koreans ended their 100-day official mourning period for Kim Jong-il, who died of a heart attack in December. Over the past three months the regime has sought to strengthen the position of Kim's youngest son and successor, Kim Jong-un, culminating in controversial rocket launch next month to mark the 100th anniversary of the birth of North Korea's founder, and Jong-un's grandfather, Kim Il-sung, on 15 April. Last month, Jong-un visited Panmunjom, the so-called "peace village" that lies inside the DMZ, and has made several high-profile inspections of military units around North Korea. Next month, the country's supreme people's assembly and ruling Workers' party are expected to give Jong-un the titles of chairman of the defence commission and secretary general of the party in an attempt to consolidate his grip on power. Obama, Chinese president Hu Jintao and dozens of other world leaders will meet in the South Korean capital Seoul on Monday to discuss ways to prevent nuclear terrorism. Iran and North Korea are not on the official agenda, but are expected to dominate bilateral discussions on the sideline of the two-day summit. North Korea insists that it has the right to launch what it describes as an observation satellite, and accused the international community of hypocrisy. "If there are any sinister attempts to deprive the (North) of its independent and legitimate right and impose unreasonable double standards, this will inevitably compel us to take countermeasures," the North's foreign ministry said in a statement. The launch will be part of attempts to portray the North as a "strong and prosperous nation" to coincide with Kim Il-sung's centenary, despite reports of widespread food shortages and malnutrition, particularly among children. Obama, who peered through binoculars into North Korea from an observation post located just 100 yards from the demarcation line, told about 50 US troops: "You guys are at freedom's frontier. The contrast between South Korea and North Korea could not be clearer, could not be starker, both in terms of freedom and in terms of prosperity. Japan's Kyodo news agency quoted unnamed sources as saying that the main portion of the three-stage rocket had been taken to a launch pad in North Phyongan province in the country's north-west. The foreign ministry in Pyongyang said preparations to put an observation satellite into orbit had entered a "full fledged stage of action." The US, Japan and other countries in the region say the launch would violate a UN ban on nuclear and missile activity because the same technology could be developed to deliver nuclear warhead on a long-range missile. The launch could ruin a deal reached last month that would give the North access to US food aid in return for stopping missile tests and ending its uranium enrichment programme. Australia and the Philippines have voiced concern about the rocket's trajectory, while Japan has threatened to shoot it down if it threatens its territory . Even China, the North's main benefactor, has urged its ally to "stay calm, exercise restraint and avoid escalation."