U.S. President Barack Obama made his first visit to the heavily fortified border with North Korea on Sunday as the communist nation is believed to have moved a long-range rocket to a launch site for final preparations for a threatened liftoff.

Obama's visit to the Demilitarized Zone separating the two Koreas is seen as a gesture underscoring Washington's security commitment to South Korea at a time of high tensions over the North's plan to launch what it claims is a satellite-carrying rocket next month.

Few believe the claim as Pyongyang has used such launches to disguise banned missile tests.

Military officials in Seoul said the North is believed to have brought the main body of a long-range rocket to the launch site in Dongchang-ri in the country's northwest, gearing up preparations to fire the rocket off between April 12 and 16 to coincide with the 100th anniversary of the April 15 birth of its founding father Kim Il-sung.

Obama arrived in Seoul earlier in the day for summit talks with South Korean President Lee Myung-bak and to attend the Nuclear Security Summit, a global anti-nuclear terrorism conference established under Obama's initiative. The inaugural summit was held in Washington in 2010.

"You guys are at freedom's frontier," Obama told a gathering of American troops during the visit to the DMZ, according to a pool report. "The contrast between South Korea and North Korea could not be clearer, could not be starker both in terms of freedom but also in terms of prosperity."

Dressed in a dark windbreaker jacket, Obama spent about 10 minutes at an observation post in the DMZ, looking at North Korea through field glasses and asking his military escorts where the demarcation line was in various directions, and the size of a North Korean village about 12 kilometers away.

About 28,500 U.S. troops are stationed in South Korea, a legacy of the 1950-53 Korean War.

Obama told the gathering of about 50 American troops that South Korean President Lee Myung-bak once confided to him that he was able to rise from poverty as a child to a successful career thanks in large part to America's military aid and support, drawing cheers from the soldiers.

Obama's meeting with Lee, set for later Sunday, is expected to focus on the rocket tensions.

The two sides are expected to reaffirm that the planned rocket launch is a grave provocation that violates a U.N. Security Council resolution banning the communist nation from any ballistic missile activity. They are scheduled to hold a joint news conference after the summit.

The North's rocket launch plan has sparked strong international denunciations as it came just weeks after Pyongyang reached a rare deal with Washington under which the country agreed to put a moratorium on nuclear and missile tests, freeze uranium enrichment and accept U.N. nuclear monitors in return for 240,000 tons of food aid.

The U.S. has said the planned liftoff, if pushed ahead, would be a deal breaker.

North Korea claims that it has a sovereign and legitimate right to launch a satellite and its "space" program has nothing to do with the deal with the U.S. On Friday, Pyongyang said the launch plan has entered "a full-fledged stage of action."

The North also warned it will take the "strongest countermeasures which no one can imagine" if South Korea "dares find fault with its nuclear deterrent and satellite launch and kick off" an anti-Pyongyang racket at next week's Nuclear Security Summit in Seoul.

The Lee-Obama meeting on Sunday is the first between them since the death of North Korean leader Kim Jong-il in December. The late leader's youngest son, Jong-un, took over power in the North.

On Monday, Obama plans to deliver a speech to college students before attending the nuclear summit that brings together more than 50 leaders and heads of major international organizations for discussions mainly on keeping nuclear material out of the hands of terrorist groups.

North Korea has denounced the Seoul security summit as an attempt to put pressure on it to give up its nuclear programs. The country has warned that if the Seoul summit produces any statement critical of its planned rocket launch, it will consider it a "declaration of war."

The North's missile program has long been a regional security concern, along with its nuclear programs. The country is believed to have advanced ballistic missile technology, though it is still not clear whether it has mastered the technology to put a nuclear warhead on a missile.

Pyongyang has carried out two nuclear tests, first in 2006 and then in 2009. (Yonhap)