US President Barack Obama visited the tense inter-Korean border Sunday, calling it "freedom`s frontier", as he weighs a response to North Korea`s planned rocket launch.

Obama is visiting South Korea to attend a 53-nation summit starting Monday on combating atomic terrorism. But the nuclear standoff with North Korea and Iran, and the upcoming launch, are set to dominate sideline discussions.

Obama told US troops in a dining hall at Camp Bonifas near the border they are part of a "long line" of soldiers who have enabled South Korea to prosper.

"You guys are at freedom`s frontier," he 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."

The US bases 28,500 troops in the South. The White House said the president`s visit to the Demilitarised Zone surrounding the border was to express his appreciation to them and stress commitment to the South`s security.

Obama, standing behind a bulletproof glass screen, peered deep into North Korea through powerful binoculars and chatted to US military officers.

A huge North Korean 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 and successor Kim Jong-Un.

A deal with the US announced last month, under which the North would observe a partial nuclear freeze and missile test moratorium in return for food aid, raised cautious hopes of eased tensions.

But less than three weeks later, the North announced it would launch a satellite between April 12 and 16 -- a plan that the US says would breach the February deal as well as UN resolutions.

Pyongyang conducted atomic weapons tests in 2006 and 2009. The US 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.

The North says there are dozens of peaceful satellite launches worldwide every year and any attempt to block its own operation would bring unspecified "counter-measures". It insists its plan does not breach the US deal.

Obama, wearing a brown leather jacket and beige slacks, landed by helicopter at Camp Bonifas, named after a US officer axed to death by North Korean troops during a clash inside the DMZ in 1976. 

His motorcade to the observation post followed a road mined on both sides,with rolls of barbed wire and tank traps.

Obama was to meet his close friend President Lee Myung-Bak in Seoul later Sunday for talks followed by a joint press conference.

He will hold talks Monday with 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 programme.

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.

The project is close to the heart of Obama, who chaired the first such summit in Washington in 2010. Experts say significant progress has been made since then, though much more needs to be done.

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

Source: ANTARA