U.S. President Barack Obama said North Korea's nuclear program poses a "grave threat" to the world. Mr. Obama and visiting South Korean President Lee Myung-bak told reporters that new responses must be found.
In the White House Rose Garden, the two leaders said that last week's U.N. resolution against North Korea developing nuclear weapons must be enforced.
Presidents Obama and Lee, standing side-by-side, agreed that they would not, under any circumstances, allow North Korea to possess nuclear weapons.
Mr. Obama said threats and belligerence would not earn prestige and respect for the communist North, and that Pyongyang has an alternative.
"I want to be clear that there is another path available to North Korea - a path that leads to peace and economic opportunity for the people of North Korea, including full integration into the community of nations," he said.
Mr. Obama said he and Mr. Lee agreed that Pyongyang can reach that destination only through peaceful negotiations.
Mr. Lee said his country, Japan, China, Russia and the United States are united in refusing to allow North Korea to continue its behavior.
"And now, the North Koreans will come to understand that this is different - that they will not be able to repeat the past or their past actions and strategies," said Mr. Lee.
Mr. Obama echoed the South Korean president's view that it is time to end the pattern of North Korea threatening its neighbors, then being rewarded for ending that behavior.
"The message we are sending, and when I say 'we' - not simply the United States and the Republic of Korea, but I think the international community - is we are going to break that pattern," he said.
The North Koreans have conducted two underground nuclear tests. And U.S. officials say Pyongyang appears to be preparing for a third.
President Obama said Pyongyang's history of threatening its neighbors and proliferating nuclear technology means it should not be recognized as a legitimate nuclear power.
Mr. Lee also called for the North Korean government to release two American journalists and a South Korean worker jailed in the North.