SEOUL (Reuters) - U.S. President Barack Obama urged China on Sunday to use its influence to rein in North Korea instead of "turning a blind eye" to its nuclear defiance, and warned of tighter sanctions if the reclusive state goes ahead with a rocket launch next month."North Korea will achieve nothing by threats or provocations," a stern-faced Obama said after a tour of the heavily fortified border between the two Koreas resonant with echoes of the Cold War.
Such a launch would only lead to further isolation of the impoverished North, which much show its sincerity if on-again-off-again six-party aid-for-disarmament talks are to restart, Obama told a news conference in the South Korean capital.
Seoul and Washington say the launch will be a disguised test of a ballistic missile that violates Pyongyang's latest international commitments. North Korea says it merely wants to put a satellite into orbit.
Even as Obama warned North Korea of the consequences of its actions, he spoke bluntly to China, the closest thing Pyongyang has to an ally, of its international obligations.
Obama said Beijing's actions of "rewarding bad behavior (and) turning a blind eye to deliberate provocations" were obviously not working, and he promised to raise the matter at a meeting with Chinese President Hu Jintao in Seoul on Monday.
"I believe that China is very sincere that it does not want to see North Korea with a nuclear weapon," he told a news conference in Seoul before a global summit on nuclear security. "But it is going to have to act on that interest in a sustained way."
It was Obama's sharpest message yet to China to use its clout with North Korea in a nuclear standoff with the West, and dovetails with recent calls for Beijing to meet its responsibilities as a rising world power.
In an election year when Republicans have accused Obama of not being strong enough with Beijing, talking tough on China is seen as a potential vote-winner after three years of troubled diplomacy in dealings with Iran, Afghanistan and Pakistan.
China is host to the six-party talks, which involve Japan and Russia as well as the two Koreas and the United States.
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 hall at one of the world's most heavily fortified frontiers.
He spent about 10 minutes on a camouflaged viewing platform at the DMZ, talking with some of the soldiers on guard and peering with binoculars across the border into North Korea as flags flapped loudly in the brisk, cold wind.
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.
The 4-km (2.5-mile) wide DMZ 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.
ROCKET LAUNCH CONDEMNED
Washington has condemned next month's planned rocket launch as a violation of North Korea's promise to halt long-range missile firings, nuclear tests and uranium enrichment in return for a resumption of food aid.
Obama said that if the North goes ahead with the rocket launch, a February food aid deal could fall apart and Pyonygang could face a tightening of international sanctions.
Obama said he was sympathetic to China's concerns that too much pressure on North Korea could create a refugee crisis on its borders, but insisted Beijing's approach over the decades had failed to achieve a "fundamental shift" in Pyongyang's behavior.
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.
North Korea's defiance is clouding Obama's much-touted nuclear disarmament agenda, which is also being challenged by Iran's persistence with nuclear research in the face of sanctions and international criticism.
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.
NORTH KOREA MOURNS
Obama's visit coincided with the end of the 100-day mourning period for 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, Obama told reporters it was difficult to get an accurate 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 in early March. 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 Jack Kim in Seoul; Editing by Jeremy Laurence and Nick Macfie)