The announcement — which has not yet been confirmed by international experts — follows a magnitude 5.1 earthquake that shook the rogue nation's nuclear test site.
The state news agency quoted Kim Jong Un saying North Korea was ready to detonate an atomic bomb or a more powerful hydrogen bomb. Experts, however, say it's unlikely the country has that capability.
President Obama is hosting South Korea's president at the White House to reaffirm one of America's longest running alliances in Asia. The long-vexing question on the agenda: North Korea.
North Korea has been holding South Korean national Joo Won-moon since April, after he admitted to illegally crossing the border in hopes of a "great event."
The restart of a reactor that produces weapons-grade plutonium was announced by the Korean Central News Agency, which also says North Korea is ready to use nuclear weapons "any time."
U.S. Ambassador Mark Lippert was at a performing arts center in Seoul preparing to give a speech on relations between the U.S. and the two Koreas when he was attacked.