US President Barack Obama paid a visit on Sunday to what he called “freedom’s frontier”.
He flew into a US base on the border between North and South Korea in a show of solidarity with Seoul.
Obama told US troops in a dining hall at Camp Bonifas near the border they are part of a “long line” of soldiers who have enabled South Korea to prosper.
“You guys are at freedom’s frontier,” he said. “The contrast between South Korea and North Korea could not be clearer, could not be starker.
“I could not be prouder of what you do.”
Obama talked with US officers before peering through binoculars into North Korea.
The two Koreas are technically still at war because no peace treaty was signed at the end of their three-year conflict in 1953.
American troops have been based South Korea ever since.
Obama is in the country for a three-day global summit on nuclear security to be held in the South Korean capital that starts Monday.
North Korea’s nuclear ambitions will feature high on the agenda.
Tensions have remained high since Kim Jong-un took power after his father’s death more than three months ago.
A deal with the US announced last month, under which the North would observe a partial nuclear freeze and missile test moratorium in return for food aid.
Earlier this month, Pyongyang angered South Korean and its allies when it announced it would launch a rocket-powered satellite in mid-April.