By Alister Bull
CAMP BONIFAS, South Korea (Reuters) - U.S. President Barack Obama on Sunday urged China to use its influence to stop North Korea's "bad behavior" in a nuclear standoff with the West and hinted at tougher sanctions if the reclusive state goes ahead with a rocket launch next month.
Such a launch would only further isolate the North, which much show its sincerity if on-again-off-again six-party aid-for-disarmament talks are to restart, Obama said.
Seoul and Washington say the launch is a disguised test of ballistic missile. North Korea says it merely wants to put a satellite in orbit.
"They need to understand that bad behavior will not be rewarded," he told a news conference in Seoul after arriving in Seoul to attend a global summit on nuclear security.
Obama urged China to be more forceful and said he would raise the subject at a meeting with Chinese President Hu Jintao on Monday.
"I believe that China is very sincere that it does not want to see North Korea with a nuclear weapon," he said. "But it is going to have to act on that interest in a sustained way."
Obama earlier visited a U.S. base on the edge of the Demilitarized Zone (DMZ) as a solemn North Korea came to a halt to mark the 100th day after "dear leader" Kim Jong-il's death.
"You guys are at freedom's frontier," Obama, wearing an Air Force One bomber jacket, told about 50 troops crammed into the Camp Bonifas mess at one of the world's most heavily fortified frontiers.
"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."
He spent about 10 minutes on a camouflaged viewing platform at the DMZ, talking with some of the soldiers on guard as the flags of the United States, South Korea and the United Nations flapped loudly in the brisk, cold wind.
Obama's tour, which followed in the footsteps of White House predecessors and bristled with Cold War symbolism, came amid rising concern over the planned rocket launch.
ROCKET LAUNCH CONDEMNED
Washington has condemned the launch plan as a violation of North Korea's promise to halt long-range missile launches, nuclear tests and uranium enrichment in return for food aid.
South Korea's Yonhap news agency quoted a military official on Sunday as saying the main body of the rocket had been moved to the launch site on North Korea's west coast. The launch will coincide with big celebrations marking the 100th anniversary of the birth of the state's founder, Kim Il-sung.
The White House cast Obama's first visit to the DMZ, which has bisected the peninsula since the end of the Korean War in 1953, as a way to showcase the strength of the U.S.-South Korean alliance and thank some of the nearly 30,000 American troops still deployed in South Korea.
Televised images of Obama venturing into the heavily mined DMZ could burnish his commander-in-chief credentials in an election year and help counter Republican accusations that he has not been tough enough on America's foes.
But North Korea's defiance is clouding Obama's much-touted nuclear disarmament agenda, which is also being challenged by Iran's continued nuclear developments in the face of sanctions and international criticism.
Obama said the window of opportunity for diplomacy with Iran was still open, and he also discussed the Syrian crisis with Turkish Prime Minister Tayyip Erdogan.
Obama will join more than 50 other world leaders on Monday for a follow-up to the inaugural nuclear security summit he organized 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.
NORTH KOREA MOURNS
Obama's first stop before holding talks with South Korean President Lee Myung-bak was the DMZ, a 4-km (2.5-mile) wide buffer that cuts through the peninsula stretching from coast to coast. Then U.S. president Bill Clinton called it the "scariest place on Earth" during a visit in 1993.
It was drawn up at the end of the 1950-53 civil conflict, which ended in a truce that has yet to be finalized with a permanent peace treaty, leaving the two Koreas in effect still at war.
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.
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 was joined by his uncle, Jang Song-thaek, and military chief Ri Yong-ho.
Standing alongside South Korean President Lee Myung-bak at media conference, Obama told reporters it was difficult to get a clear impression of how the succession process was going because it was not clear who was "calling the shots" in the North.
The young Kim himself 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".
(Additional reporting by Matt Spetalnick and Jack Kim in Seoul; Editing by Jeremy Laurence and Nick Macfie)