WASHINGTON -- Declaring North Korea a "grave threat" to the world, President  Obama on Tuesday pledged the U.S. and its allies will aggressively enforce fresh international penalties against the nuclear-armed nation and stop rewarding its leaders for repeated provocations.

In a display of unity with South Korea's leader, Obama said the world must break a pattern in which North Korea puts the globe on edge, only to put itself in line for concessions if it holds out long enough.

"We are more than willing to engage in negotiations to get North Korea on a path of peaceful coexistence with its neighbors, and we want to encourage their prosperity," Obama said in the Rose Garden alongside South Korean President Lee Myung-bak. "But belligerent, provocative behavior that threatens neighbors will be met with significant and serious enforcement of sanctions that are in place."

Obama's comments came at a time of intensifying concern, with the North stepping up its bomb-making activities and threatening war against any country that blockades its ships. Pentagon officials warned on Tuesday that North Korea's missiles could strike the U.S. within three years if its weapons growth goes unchecked.

Emboldened by fresh assurances of protection by the United States, Lee went even further in warning that North Korea's tactics will not be tolerated. Asked if he felt his country was under the threat of attack from the North, Lee said his country's alliance with the U.S. will "prevent anything from happening."

He said of the North Koreans, "They will think twice about taking any measures that they will regret."

Defiantly pursuing its nuclear ambitions, North Korea has posed a major foreign policy challenge for Obama. However, the new president has found support from the international community, including a swift resolution of sanctions imposed by the U.N. Security Council just last week.

The new punishments toughen an arms embargo against North Korea and authorize ship searches in an attempt to thwart the Koreans' nuclear and ballistic missile programs. The U.N., however, did not authorize military force to enforce the measures.

North Korea provoked that rebuke by conducting its second nuclear test on May 25, following recent missile launches that had already alarmed the world.

Beyond enforcement of the new U.N. penalties, Lee said he and Obama agreed on something more -- a push for other new policies that will "effectively persuade North Korea to irrevocably dismantle all their nuclear weapons programs." The South Korean leader said those measures will be discussed among the five nations that had been working with North Korea on disarmament until talks stalled: the U.S., South Korea, Japan, Russia and China.

He did not elaborate, and the White House had no comment on the matter.

North Korea has bargained with other countries for more than a decade about giving up its nuclear program, gaining such concessions as energy and economic aid, and then reneging.

The North is thought to have enough weaponized plutonium for at least half a dozen atomic bombs and is believed to be preparing for another nuclear test. Deepening the crisis, it responded to the new sanctions by promising to "weaponize" all its plutonium and step up its nuclear bomb-making by enriching uranium -- the first time it had acknowledged it had such a program. Both plutonium and uranium can be used to make atomic bombs.

With all that as a backdrop, Lee's treatment at the White House was meant to underscore solidarity at a perilous time.

The South Korean president was the first foreign leader in Obama's nearly five-month-old presidency to get the honor of a joint appearance in the Rose Garden. He spoke repeatedly of his nation's firm partnership with the United States and thanked the American people "for their selfless sacrifice in defending my country and its people." Obama said the friendship was anchored in democratic values, and then he turned his words on the country's northern neighbor.

"North Korea has abandoned its own commitments and violated international law," Obama said. "Its nuclear and ballistic missile programs pose a grave threat to peace and security of Asia and to the world."

Obama said that North Korea's record of threatening other countries and spreading nuclear technology around the world means it should not be recognized as a legitimate nuclear power.

At a missile defense hearing on Capitol Hill, Deputy Defense Secretary William Lynn pointed to North Korea's recent steps to speed up its long-range weapons program and agreed with Sen. John McCain, R-Ariz., that the U.S. should be prepared for a "worst-case scenario."

"We think it ultimately could -- if taken to its conclusion -- it could present a threat to the homeland," Lynn said at a Senate Armed Services Committee hearing.

The House Armed Services Committee Tuesday tentatively approved Pentagon plans to spend $9.3 billion on missile defense for the 2010 fiscal year that begins Oct. 1. That marked what Republicans called a $1.6 billion cut from what had been budgeted for missile defense during the Bush administration.

The panel's chairman, Rep. Ike Skelton, D-Mo., said the new spending plans trim some fat from missile defense budgets while still protecting the U.S. against foreign threats.

But Sen. Jeff Sessions, R-Ala., said the Obama administration's cut to missile defense was too steep at a time when North Korea and Iran are ramping up their long-range nuclear weapons programs.

At the Defense Department, press secretary Geoff Morrell declined to say when interdiction operations might begin under the new U.N. sanctions, but he said the U.S. already has enough ships and other resources in the region to do the job. Morrell was asked what the point of the activity would be -- and whether it was only a half-measure -- as long as there was no authority to forcibly board Korean ships.

"I think if the world is in agreement that we are all going to monitor and then attempt to compliantly board and attempt to then direct those ships into a port where they can then be inspected, that is real progress," he said. "That is more than what we were doing before."