Obama visits Koreas' Demilitarised Zone as tensions rise

US President Barack Obama has visited US troops stationed on South Korea's border with North Korea in a show of solidarity with Seoul, ahead of a global summit there on nuclear security.

Obama flew on Sunday by helicopter to a US base on the edge of the Demilitarised Zone (DMZ) to meet troops and get a first-hand look at one of the world's most heavily fortified frontiers.

He is expected to lobby the leaders of China and Russia at the Seoul summit to step up pressure on Pyongyang, amid rising concern over a planned North Korean rocket launch next month.

"You guys are at freedom's frontier," he told about 50 troops in the Camp Bonifas mess.

"The contrast between South Korea and North Korea could not be clearer, could not be starker, both in terms of freedom and in terms of prosperity."

Washington has condemned reclusive and impoverished North Korea's rocket launch plan, which it says will send a satellite into orbit, as a violation of its promise to halt long-range missile launches, nuclear tests and uranium enrichment.

The US warned that a deal to resume stalled food aid to the North, agreed earlier this month, could be jeopardised if Pyongyang goes ahead.

The White House cast Obama's first visit to the DMZ, the four-kilometre wide strip that has divided the peninsula since the end of the Korean War in 1953, as a way to showcase the strength of the US-South Korean alliance and thank some of the more than 20,000 US troops still deployed there.

Although the conflict between North Korea and South Korea has effectively been frozen for nearly 60 years, the countries remain technically at war because their conflict ended with a truce agreement rather than a formal peace treaty.

Obama will join more than 50 other world leaders for a follow-up to the inaugural nuclear security summit he organised in Washington in 2010 to help combat the threat of nuclear terrorism.

While North Korea and Iran are not on the guest list or the official agenda, they are expected to be the main focus of Obama's array of bilateral meetings on the sidelines of the two-day summit.

Obama's visit coincided with the end of the 100-day mourning period for the North's long-time leader, Kim Jong-il, who died in December.

Tens of thousands of people crammed into Kim Il-sung Square in central Pyongyang to mark the occasion.

Flags flew at half-mast in "every nook and corner" of the country and sirens and whistles sounded at midday, state media said. North Koreans "overcome with grief" observed three minutes' silence.

The state's new young leader, Kim Jong-un, the third member of the Kim family to rule the state, bowed before a portrait of his father at the palace where he lies in state.

He made a surprise trip to the DMZ at the start of the month. He looked across the border through binoculars and told troops to "maintain the maximum alertness since [they] stand in confrontation with the enemy at all times".