US President Barack Obama has visited the Demilitarised Zone (DMZ) separating the two Koreas, amid rising tensions over the North's planned rocket launch.
At an observation post on the volatile border, he told US troops they were "at freedom's frontier".
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 nuclear security summit in Seoul.
He told a news conference with his South Korean counterpart that North Korea would achieve nothing with "threats and provocations".
A two-day conference in the South Korean capital will be attended by leaders from more than 50 nations.
Its main focus will be preventing criminal or militant groups from acquiring nuclear weapons.
Pyongyang's nuclear programme is not officially on the agenda and North Korea is not taking part in the summit.
But American officials have made it clear that Mr Obama will be discussing the programmes of both North Korea and Iran in bilateral meetings with the Chinese and Russian presidents, the BBC's Lucy Williamson in Seoul reports.
China's state media said the North Korea rocket launch had "stolen some of the limelight" but said the summit was "not an appropriate platform" to discuss it and urged the world leaders not to be distracted.
The DMZ, a 4km (2.5-mile) wide strip of heavily mined land, has bisected the peninsula since the end of the Korean War in 1953. It is considered one of the world's most heavily-guarded border areas.
Making his first visit to the DMZ, Mr Obama told US troops serving at Camp Bonifas: "I could not be prouder of what you're doing... You guys are at freedom's frontier."
The president said the contrast between the two sides of the border "could not be starker" and paid tribute to the military personnel who had been "willing to create the space and the opportunity for freedom and prosperity".
Mr Obama then spent about 10 minutes at an observation post, looking towards North Korea through binoculars from behind bullet-proof glass.
Earlier this week, Japan said it was readying its anti-missile defences ahead of North Korea's launch, expected between 12 and 16 April.
Pyongyang says the long-range rocket - which would mark the 100th birthday of its late Great Leader Kim Il-sung - would take a new southern trajectory instead of a previous route east over Japan.
US Assistant Secretary of State Kurt Campbell has said an area between Australia, Indonesia and the Philippines could be affected by the flight path.
On Sunday, South Korean defence officials said the North had moved the main body of the rocket to a launch site in north-west of the country in preparation.
North Korea has shown a growing mastery of ballistic technology during its three previous long-range tests.
However, experts say none has succeeded in reaching orbit, and debris has fallen to earth at various stages during the launches.
Last month North Korea offered a deal involving the freezing of its nuclear and long-rang missile programmes in return for US food aid, which was seen as a signal that Pyongyang was moving towards exploring a diplomatic resolution to the stand-off.
But Washington has accused Pyongyang of acting in bad faith following the rocket launch announcement, which could put jeopardise the promised aid delivery.
Pyongyang has threatened that if South Korea discusses its nuclear programme at the Seoul summit it will retaliate with the "strongest counter-measures which no one can imagine".
Meanwhile, tens of thousands of people in North Korea have marked the official end of 100 days of mourning for late leader Kim Jong-il, who died of a heart attack in December.
Crowds gathered in the capital, Pyongyang, to pay their respects to Mr Kim and pledge loyalty to his son and successor, Kim Jong-un.