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.

Daniel Pinkston, a Seoul-based analyst for the International Crisis Group think tank, noted that the United States' first uranium bomb wasn't tested until it was dropped on Japan in August 1945.

In contrast, "a plutonium bomb generally is more sophisticated and needs to be tested before it can be used with confidence," he said.

Little concrete information is available about North Korea's uranium program and how far they've come in developing it. Oelrich estimated it is "in its infancy."

One senior South Korean official said he suspects the North has already embarked on uranium enrichment with the ultimate purpose of building bombs. He spoke on condition of anonymity, citing the sensitivity of the issue.

"I don't believe they have a commercial-scale plant up and running, and it will take them some time," Pinkston said. "However, they could cooperate with Iran and reduce the time required to build and operate a large-scale facility since Iran has made significant progress and is already operating a large facility."

North Korea and Iran are believed to be trading information about nuclear and missile technology, making proliferation a key concern.

"The more fissionable materials they have ... the more dangerous is the situation," said James Kelly, a former assistant U.S. secretary of state who confronted North Korean officials about uranium enrichment during a 2002 visit to Pyongyang.

North Korea claimed earlier this month it was "compelled to go nuclear" because of hostility from Washington.

"It has become an absolutely impossible option for the DPRK to even think about giving up its nuclear weapons," the Foreign Ministry said in a statement carried by state media, using the initials for the country's official name, the Democratic People's Republic of Korea.