At a joint news conference with Foreign Minister Fumio Kishida, Mr Kerry said North Korea had to understand by now "that its threats and provocations are only going to isolate it further and impoverish its people even further".

BBC: Kerry urges North Korea to join regional dialogue

They do not seem to understand that the United States and nations in their own region feel threatened by North Korea's nuclear program, which has in the past produced enough plutonium to build several nuclear weapons.

CNN: Behind the Scenes: Amanpour's notes from North Korea