SEOUL, South 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.

Since 2003, they have focused on persuading the North to disable a nuclear reactor north of Pyongyang, where the communist regime had been laboriously extracting plutonium, not a naturally occurring material, from spent fuel rods.

Natural uranium, on the other hand, is readily available. North Korea has said it has an estimated 26 million tons of natural uranium deposits, of which about four million tons can be economically extracted. The Washington-based Federation of American Scientists also said an estimated 4 million tons is high-quality uranium ore.

That doesn't mean North Korea can make a uranium bomb overnight. The uranium must be highly enriched first, and making enough for a bomb requires operating 1,000 to 3,000 centrifuges for a year, said Lee Choon-geun, an expert at South Korea's state-funded Science and Technology Policy Institute.

But its recent announcement suggests the country has begun heading in that direction.

And once the weapons-grade enriched uranium is in hand, it is "significantly easier" to build a bomb from it than from plutonium, said Ivan Oelrich, vice president of the Federation of American Scientists.

Uranium also can be enriched in a facility like an ordinary factory and doesn't release much heat compared with the plutonium-producing reactor at Yongbyon, north of Pyongyang. That makes it difficult for spy satellites to detect, according to South Korea's Institute of Nuclear Nonproliferation and Control.

And testing is not as essential for bombs built from uranium as for plutonium bombs. The North has conducted two nuclear tests of plutonium-made bombs, in 2006 and in May, which drew international condemnation and garnered U.N. sanctions.