Obama lashes North Korea as 'pariah state'

North Korea is a "pariah state" whose heavily militarised border with the South marks "freedom's frontier", US President Barack Obama has told American troops in Seoul.

Obama, who was wrapping up a two-day visit to South Korea, said on Saturday that Pyongyang's continued pursuit of nuclear weapons is "a path that leads only to more isolation".

His comments come after satellite images revealed the North could be preparing to carry out an atomic test - its fourth - despite stringent sanctions imposed by the international community.

They also come hours after Pyongyang state media claimed the North had been holding a young US citizen for two weeks because of his "rash behaviour" while passing through immigration.

Obama made no mention of the man whom Pyongyang identified as 24-year-old "Miller Matthew Todd", but said the North's border with its southern neighbour separates two very different places.

"The 38th Parallel now exists as much as a contrast between worlds as it does a border between nations, between a society that's open and one that is closed," he said.

"(It is a border) between a democracy that is growing and a pariah state that would rather starve its people than feed their hopes and dreams.

"Freedom is not an accident. Progress is not an accident. Democracy is not an accident.

"These are things that have to be fought for... And they've got to be tended to constantly and defended without fail. And here, on freedom's frontier, they are."

Pictures from satellites have suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels at North Korea's Punggye-ri test site.

North Korea has conducted three nuclear tests, in 2006, 2009 and 2013.

Obama blasted Pyongyang's atomic program, which he said would do the country no good.

"North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation," he said.

The Korean Central News Agency (KCNA) reported Saturday that leader Kim Jong-Un had urged his soldiers to be ready for "impending conflict with the United States".

Obama, whose four nation tour of Asia began in Japan, said the US was committed to its friends in the region and pledged the US-South Korean alliance was as "strong as it has ever been".

Obama was at the US Army's Yongsan garrison with President Park Geun-Hye to visit the command centre for joint military operations.

There are around 28,500 US service personnel in South Korea. Under present arrangements, US forces would take command of the South's military, but an agreement is due to change that in 2015.

However, on Friday Park and Obama agreed that they could reconsider that strategy and push back the handover date.

Obama left Seoul, bound for Kuala Lumpur. His Asian tour will also take him to the Philippines.