WASHINGTON- U.S. President Barack Obama and South Korean President Lee Myung-bak presented a united front to North Korea on Tuesday, saying Pyongyang must abandon its nuclear weapons program and will not be rewarded for provoking a crisis.

With Lee at his side in the White House Rose Garden, Obama said a nuclear-armed North Korea would pose a grave threat to the world and vowed that new U.N. sanctions against the reclusive communist-ruled nation would be strictly enforced.

Given the belligerent manner in which they are constantly threatening their neighbors, I don't think there's any question that that would be a destabilizing situation that would be a profound threat to not only the United States' security, but to world security, Obama said.

He promised to end a cycle of allowing impoverished North Korea to create a nuclear crisis, then granting concessions in the form of food, fuel and other incentives to get Pyongyang to back down, only to later see it renege on its promises.

This is a pattern they've come to expect, Obama said. We are going to break that pattern.

While talking tough, Obama -- who took office in January pledging a new approach of talking to America's enemies -- also extended an olive branch.

I want to be clear that there is another path available to North Korea ... including full integration into the community of nations, Obama said. That destination can only be reached through peaceful negotiations that achieve the full and verifiable denuclearization of the Korean peninsula.

North Korea, which last month conducted a nuclear explosion and missile tests in defiance of international pressure, said at the weekend it would start a uranium enrichment program and weaponize all its uranium in response to new U.N. sanctions.

Lee said the U.N. Security Council's vote last week to expand sanctions on North Korea showed the global community's firm resolve.

He said South Korea, along with the United States, Japan, China and Russia -- members of stalled six-party talks with Pyongyang -- will be seeking new measures to get the North to irrevocably dismantle all nuclear weapons programs. The White House declined to say what actions were being considered.

The South Korean leader has followed a tough line on North Korea, even before Pyongyang raised tensions in recent weeks by test-firing missiles, restarting a plant to produce arms-grade plutonium and conducting a nuclear test on May 25.

As a stark message to Pyongyang, Obama re-committed to Washington's defense of South Korea, including keeping it under America's nuclear umbrella.

NORTH KOREAN HEIR

Japan's Asahi newspaper reported on Tuesday that the youngest son of North Korean leader Kim Jong-il secretly visited China last week and his hosts were told he had been appointed heir to the ruling family dynasty.

The report, citing unidentified informed sources, said Kim Jong-un met Chinese President Hu Jintao and other leaders of the ruling Communist Party when he flew to Beijing around June 10.

Analysts have said North Korea's recent nuclear test and other belligerent acts may be aimed at a domestic audience, with the elder Kim trying to bolster his position at home to secure the succession of his youngest son. The 67-year-old leader is believed to have suffered a stroke last year.

An aide to Jong-un told Chinese officials the younger Kim had been appointed heir and that he held an important post in the ruling Korean Workers' Party, Asahi said.

A Chinese Foreign Ministry spokesman said he had no knowledge of the reported visit. North Korean state media has never told the country's public that leader Kim has children, let alone reported on a trip by one of them.

LOYALTY PLEDGE

Jong-un is the Swiss-educated third son of Kim Jong-il and was born in 1983 or 1984. Earlier this month South Korean media, quoting informed sources, said Pyongyang had asked the country's main bodies and overseas missions to pledge loyalty to him, indicating he will take over from his father.

China is the closest thing North Korea has to an ally, and in theory Beijing wields more influence over Pyongyang than any other power, but experts say the relationship is brittle and China actually has limited room for maneuver.

Hu apparently asked North Korea not to go ahead with another nuclear test or test-fire an intercontinental ballistic missile, Asahi reported. Jong-un was believed to have asked China for emergency energy and food aid, the newspaper said.

In a development on a case that has caused further tensions with Washington, the official North Korean news agency KCNA gave its first details of the case of two U.S. journalists it sentenced to 12 years of hard labor last week, saying they were trying to slander the state.

At the trial the accused admitted that what they did were criminal acts committed, prompted by the political motive to isolate and stifle the socialist system of the DPRK (North Korea) by faking up moving images aimed at falsifying its human rights performance and hurling slanders and calumnies at it,

We are following with a high degree of vigilance the attitude of the U.S. which spawned the criminal act against the DPRK, KCNA said. The reporters are Laura Ling and Euna Lee, both in their 30s and working for media outlet Current TV.

(Additional reporting by Ross Colvin, David Alexander and Jeff Mason in Washington, Yuko Kubota and Yoko Nishikawa in Tokyo, Jon Herskovitz, Jack Kim and Christine Kim in Seoul, Emma Graham-Harrison and Lucy Hornby in Beijing; Editing by David Storey and Frances Kerry)

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