U.S. Secretary of State John Kerry told reporters that the North has a clear course of action available to it and will find "ready partners" in the U.S. if the reclusive communist government follows through.
Japan's foreign minister, Fumio Kishida, who appeared with Kerry at a news conference, said North Korea must honor its commitment to earlier deals regarding its nuclear and missile programs and on returning kidnapped foreigners.
The officials opened the door to direct talks with Pyongyang if certain conditions are met, and agreed on the need to work toward a nuclear-free North Korea.
Saturday, Kerry and Chinese leaders in Beijing pledged to resolve the North Korean nuclear situation in a peaceful manner. Kerry said gaining China's commitment was no small matter, given Beijing's historically strong military ties to the North.
The issue has climbed in urgency in recent months, given North Korea's tests of a nuclear device and intercontinental ballistic missile technology, and its increasingly brazen threats of nuclear strikes against the United States.
U.S. and South Korean officials believe North Korea may deliver another provocation in the coming days with a mid-range missile test.
Given their proximity and decades of hostility and distrust, Japan and South Korea have the most to fear from the North's unpredictable actions.
Kerry said the U.S. would defend both its allies at all cost.
The Associated Press contributed to this report.