US President Barack Obama has visited the Demilitarised Zone (DMZ) separating the two Koreas, amid rising tensions over the North's planned rocket launch.
He went to an observation post on the volatile border, and met US troops. He will hold talks in Seoul later.
The US has voiced concern that the rocket launch due in April is a pretext for a missile test. Pyongyang says it wants to put a satellite into orbit.
On Monday, Mr Obama will attend a security summit in Seoul.
The conference in the South Korean capital will be attended by leaders from more than 50 nations, including New Zealand Prime Minister John Key.
The BBC reports its main focus will be preventing criminal or militant groups from acquiring nuclear weapons.
Pyongyang's nuclear programme is not officially on the agenda.
But American officials have made it clear that President Obama will be discussing the programmes of both North Korea and Iran in bilateral meetings with the Chinese and Russian presidents.
North Korea is not taking part in the summit.
It was Mr Obama's first visit to the DMZ, which has bisected the peninsula since the end of the Korean War in 1953.
The president told US troops serving there that they were protecting "freedom's frontier".