US Defence Secretary Chuck Hagel said he took North Korea's nuclear threats "seriously" as the US sent a missile defence system to Guam as a "precaution."
Hagel added that North Korea now had nuclear and missile delivery capacity and posed a "real and clear" danger to the US and its allies.
Through her tears, a North Korean woman told me how she had to escape: risk death while running across the border or die of starvation.
Memories of the Korean war are never far away in South Korea, where residents have grown used to living with the constant threat of war.
North Korea said it will withdraw workers from the Kaesong factory park run jointly with South Korea and suspend all operations there.