US president Barack Obama and South Korea's new leader have warned North Korea against further nuclear provocations, with Mr Obama declaring that the days when Pyongyang could "create a crisis and elicit concessions" were over.
The meetings between Mr Obama and President Park Geun-hye followed months of increased tensions on the Korean Peninsula.
North Korea conducted an underground atomic test in February and had appeared ready for another. New US intelligence assessments also showed for the first time that North Korea may have the know-how to launch a nuclear-armed missile, though American officials say Pyongyang still appears to lack the capability to carry out an attack.
Before the talks, the North appeared to send mixed messages. US officials said Pyongyang removed from a launchpad a set of medium-range ballistic missiles that had been readied for possible test-firing. But North Korea also warned the US and South Korea that it would retaliate if joint military exercise between the two allies resulted in any shells landing on its territory.
Analysts see some of North Korea's recent bluster as an attempt by the country's new leader, Kim Jong Un, to establish himself as a power player, both within his own country and in the international community. Mr Obama said he knew little about Kim personally and had never spoken to him, but added that his actions were leading him down a dead end.
"There's going to have to be changes in behaviour," Mr Obama said. "We have an expression in English, 'Don't worry about what I say, just watch what I do.'"
Mr Obama also disputed the notion that his cautious response to reported chemical weapons use in Syria - a move he had said would cross a "red line" - could embolden North Korea's unpredictable young leader and other US foes.
"Whether it's bin Laden or Gaddafi, if we say we're taking a position, I would think at this point the international community has a pretty good sense that we typically follow through on our commitments," he said, referring to al Qaida commander Osama bin Laden and former Libyan leader Muammar Gaddafi, both of whom were killed during Mr Obama's watch.
Speaking at a joint news conference at the White House, Mr Obama and Ms Park warned Pyongyang of unspecified consequences if it pressed ahead with provocative actions, with Mr Obama vowing to protect the US and its allies using both "conventional and nuclear forces".
But, in keeping with their countries' long-standing policies, the two leaders left open the possibility of direct negotiations should the North signal its readiness to end its nuclear pursuits or take other meaningful actions. "Should North Korea choose the path to becoming a responsible member of the community of nations, we are willing to provide assistance, together with the international community," Ms Park said.