U.S. President Barack Obama said on Tuesday that North Korea's nuclear posed a grave threat not only to the United States, but to world security.

He vowed that new U.N. sanctions against the reclusive communist nation would be strictly enforced, speaking at a news conference after meeting South Korean President Lee Myung-bak at the White House.

Obama promised to end a cycle of allowing impoverished North Korea to create a nuclear crisis by granting concessions in the form of food, fuel and other incentives to get Pyongyang to back down, then 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, he also extended an olive branch to North Korea since he has taken office in January, pledging a new approach of talking to America's enemies.

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, he added.

North Korea said at the weekend it would start a uranium enrichment program and weaponize all its uranium in response to new U.N. sanctions.