North Korea ? After repudiating negotiations on dismantling its plutonium-based nuclear program, North Korea admitted this month to having an even more worrying way to make bombs.

Following nearly seven years of adamant denials, North Korea announced it can enrich uranium — a simpler method of building nuclear weapons than reprocessing plutonium. Uranium can be enriched in relatively inconspicuous factories that can better evade spy-satellite detection, and uranium bombs may work without test explosions.

The admission — made in a threatening response to a June 12 U.N. Security Council resolution punishing Pyongyang for an underground plutonium bomb test last month — poses a new challenge to the U.S., China, South Korea, Russia and Japan as they seek to stem the reclusive country’s atomic ambitions.