Obama visits Korean DMZ ahead of nuclear summit in Seoul U.S. President Barack Obama arrives at Osan Air Base on March 25, 2012 in Osan, South Korea. President Obama makes his first trip to the Korean Demilitarized Zone The president is in South Korea for an international nuclear summit He is scheduled to meet South Korea's president Seoul, South Korea (CNN) -- President Barack Obama used his first visit to the demilitarized zone that splits the Korean peninsula to peer through binoculars into North Korea where flags flew at half-staff to mark the 100-day anniversary of the death of Kim Jong Il. The stop at the border Sunday marked the start of a three-day trip to South Korea where the American president is to attend an international nuclear summit. Top officials from 54 countries including China and Russia will attend the summit. Obama's visit to the demilitarized zone was to meet with some of the 28,500 U.S. troops stationed in South Korea. But the stop also appears to have been intended as a message to North Korea's new leader, Kim Jong Un, that the United States remains committed to South Korea and in its standoff with the country over its development of nuclear power. "The contrast between South Korea and North Korea could not be clearer, could not be starker, both in terms of freedom and prosperity," Obama told troops at a base near the DMZ. It was Obama's first trip to the demilitarized zone, though he has made two previous trips to South Korea as president. Secretary of State Hillary Clinton paid a visit to the area in 2010. Across the border, North Koreans were mourning the death of Kim. Kim's son, who took over from his father following his death in December, and senior party officials visited the Kumsusan Palace of the Sun in Pyongyang to observe a moment of silence, said North Korea's state-run Korean Central News Agency (KCNA). The palace serves as the burial site for Kim Il-sung, the founder of North Korea, and his son, Kim, who succeeded him as the country's ruler. In South Korea, Obama is expected to hold a bilateral meeting with his Seoul counterpart Lee Myung-bak. KCNA cited a spokesman from the Secretariat of the Committee for the Peaceful Reunification of Korea as denouncing the South for working to turn the summit "into a platform for (an) international smear campaign" against the North. Pyongyang announced this month it would carry out a "satellite launch" in mid-April to commemorate the centenary of the birth of Kim Il Sung, the country's founder. The date of Obama's visit "is virtually two years to the day" since the sinking of the South Korean warship, the Cheonan, which left 46 Southern sailors dead, said Daniel Russel, director for Japan, South Korea, and North Korea at the U.S. National Security Council. CNN's Paula Hancocks, Shruti Pant, Jethro Mullen and Bob Kovach contributed to this report.