US President Barack Obama peered deep into North Korea Sunday, as he contemplated Pyongyang's planned rocket launch and his first showdown with untested leader Kim Jong-Un.

Obama stood behind bulletproof glass at the tense inter-Korean border scanning the wooded hills of the Stalinist North, then gazed out at a distant North Korean flag, flying at half mast in memory of late strongman Kim Jong-Il.

The president told some of the 28,500 US soldiers helping to guard the world's last Cold War land frontier, that they were part of a "long line" of Americans who had granted South Korea the chance to prosper.

"You guys are at freedom's frontier," Obama 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's visit was meant as a strong sign of support for South Korea, with which he has formed perhaps his closest bond with an Asian power since taking office in 2009 and reorienting US foreign policy towards the Pacific.

It comes as tensions rise with Pyongyang, which says it will launch a satellite next month.

Washington says such a move would breach a deal to offer food aid in return for a partial nuclear freeze and missile test moratorium, as well as UN resolutions.

North Korea has transported the rocket's main body to a site in the far northwest of the country in preparation for next month's launch, Seoul's defence ministry said Sunday.

The North has announced it will fire the rocket to put a satellite into orbit between April 12-16 to mark the 100th anniversary of the birth of founding president Kim Il-Sung.

Washington says the launch is a disguised missile test.

Obama travelled by helicopter to the Demilitarised Zone between the two Koreas at Camp Bonifas, named after a US officer axed to death by North Korean troops during a clash inside the DMZ in 1976.

He will be in Seoul for a 53-nation summit on combating atomic terrorism opening Monday but overshadowed by the nuclear and rocket standoff with Pyongyang and a separate nuclear crisis with Iran.

Casually dressed in a brown leather jacket, Obama trained powerful binoculars over the border as he stood on a windy ridge from which he could see 17 kilometres (10 miles) into the impoverished North.

Observation Post Ouellette marks the closest post to the demarcation line between the two Koreas. It is reached by a road through a live minefield and tank traps meant to deter any move south by the North's million-strong army.

Obama followed former presidents Ronald Reagan, Bill Clinton, and George W. Bush to the DMZ, and like his recent predecessors has been confounded by the isolated North's volatile behaviour and nuclear brinkmanship.

The flag displayed at a village in the North's section of the DMZ was flying at half-mast to mark the 100th day since the death of longtime leader Kim Jong-Il. The North held a national memorial service Sunday.

Tensions have remained high under Kim's young son Kim Jong-Un.

Pyongyang conducted atomic weapons tests in 2006 and 2009. The United States and other nations say its proclaimed satellite launches are really tests of missiles which could deliver a nuclear warhead at some point in the future.

Obama was to meet his close friend President Lee Myung-Bak in Seoul later Sunday for talks followed by a joint press conference on a trip marking a rare foray outside the United States this year as his re-election bid gathers pace.

He also planned talks with Turkish Prime Minister Recep Tayyip Erdogan, expected to centre on so-far futile global efforts to halt the Syrian government's brutal crackdown on rebels.

Obama will Monday meet China's President Hu Jintao and outgoing Russian President Dmitry Medvedev on the sidelines of the summit.

China is the North's key ally. It and Russia -- along with South Korea, Japan and the United States -- are involved in stalled negotiations which began in 2003 on scrapping Pyongyang's nuclear weapons programme.

China's state news agency Sunday urged world leaders not to stray off topic by addressing the North's nuclear programme at the gathering.

The two-day Nuclear Security Summit will focus on minimising the threat of nuclear-armed terrorism and securing or destroying the world's supplies of plutonium and highly enriched uranium.

North Korea has denounced it as a "burlesque" intended to rally world opinion against its nuclear programme.