Video: Obama peers over at North Korea
Arriving on the eve of a global summit on nuclear security hosted by South Korea, Mr Obama flew by helicopter to a US base on the edge of the Demilitarized Zone (DMZ) to meet troops stationed there and get a first-hand look at one of the world's most heavily fortified frontiers.
He spent about 10 minutes on a viewing platform at the DMZ, talking with some of the soldiers on guard as the flags of the United States, South Korea and the United Nations flapped loudly in the brisk, cold wind.
Mr Obama peered through binoculars from behind bullet-proof glass across to North Korea, pointing at something in the distance. The North Korean flag flew at half-mast for former leader Kim Jong-il.
His visit to South Korea came amid rising concern over a planned North Korean rocket launch next month that threatens to derail a deal to resume US food aid.