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. But its message of international cooperation has been overshadowed by North Korea's announcement last week that it is planning to carry out a rocket-powered satellite launch in April.
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.
"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 told troops at a base near the DMZ.
"And the contrast between South Korea and North Korea could not be clearer, could not be starker, both in terms of freedom but also in terms of prosperity."
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.
President Lee has already said he will use the summit to drum up international support against the actions of his northern neighbor.
South Korea has said it considers the satellite launch an attempt to develop a nuclear-armed missile, while the United States has warned the move would jeopardize a food-aid agreement reached with Pyongyang in early March.
North Korea says it has a right to a peaceful space program and has invited international space experts and journalists to witness the launch.
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.
Seoul's nuclear summit will be the second after Obama hosted the first meeting in Washington in 2010. He initiated the biennial summit after presenting his vision of a nuclear-free world in Prague in April 2009.
The official agenda will deal with nuclear terrorism and how to secure the world's nuclear material.
Although North Korea is not on the agenda, it is likely to be discussed on the sidelines.
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.
Using ballistic missile technology, however, is in violation of U.N. Security Council Resolution 1874 and against a deal struck with the United States earlier this month that it would not carry out nuclear or missile tests in return for food aid.
Pyongyang has said it will see any critical statement of its nuclear program as "a declaration of war."
Concerns about Iran's nuclear program, again not on the official agenda, will also be discussed in bilateral meetings between leaders.
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.
South Korea says a North Korean torpedo attack was to blame for the ship's sinking. The North has denied the accusation.
CNN's Paula Hancocks, Shruti Pant, Jethro Mullen and Bob Kovach contributed to this report.