U.S. President Barack Obama has visited the Demilitarized Zone (DMZ) that separates North and South Korea.
Obama went to an observation post on the border, and met with U.S. troops. By visiting the DMZ, Obama emulated former presidents Ronald Reagan, Bill Clinton, and George W. Bush.
Obama is in Korea to attend an international summit in Seoul on the potential threat of nuclear terrorism and how to combat it.
More than 50 world leaders are expected to attend the gathering on March 26 and March 27.
Obama was due to meet to meet the president of South Korea, Lee Myung-Ba, in the South Korean capital on March 25 for talks followed by a joint press conference.
He was also scheduled to hold talks with Turkish Prime Minister Recep Tayyip Erdogan, which are expected to focus on faltering global efforts to halt the Syrian government's brutal crackdown on rebels.
On March 26, Obama will meet China's President Hu Jintao and outgoing Russian President Dmitry Medvedev on the sidelines of the Seoul summit.
China is a key ally of North Korea. Along with South Korea, Japan and the United States, China and Russia are involved in stalled negotiations, which began in 2003, on scrapping Pyongyang's nuclear weapons program.
The two-day Nuclear Security Summit will focus on minimizing the threat of nuclear-armed terrorism and securing or destroying the world's supplies of plutonium and highly enriched uranium.
North Korea has denounced the Seoul summit.
Obama's visit comes amid fresh tension with Pyongyang over a planned long-range rocket launch.
Reports say that North Korea has transported the main body of a long-range rocket to a site in the far northwest of the country in preparation for the launch.
Pyongyang has announced that, between April 12 and April 16, it will fire the rocket to put a satellite into orbit to mark the 100th anniversary of the birth of North Korea's founding president Kim Il-Sung.
On March 25, North Korea urged its people to rally behind new leader Kim Jong Un.
Tens of thousands gathered in a central square to observe the end of a 100-day mourning period following the death of his father, Kim Jong Il.
Washington is concerned about the launch and suspects it is a disguised missile test.
The United States has said the plan jeopardizes a deal to provide American food aid to North Korea.
With reporting by AP, AFP, and Reuters