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.

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, where he addressed some of the 28,500 US soldiers helping to guard the world's last Cold War land frontier.

"You guys are at freedom's frontier," Obama told the troops. "The contrast between South Korea and North Korea could not be clearer, could not be starker.

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.

A South Korean official quoted by Yonhap news agency said officials in Seoul and Washington believed North Korea had transported the main body of a long-range rocket to its northwestern launch site of Tongchang-ri.

The North has announced it will fire the rocket to put a satellite into orbit between April 12-16, marking 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 demilitarized 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.

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.

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

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

North Korea has denounced the Seoul summit as a "burlesque" intended to rally world opinion against its nuclear program.