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.

The statement did not make clear whether the regime has secretly built up the program over the past seven years that it has denied its existence — or started it recently.

"The process of uranium enrichment will be commenced," the June 13 statement said. "Pursuant to the decision to build its own light-water reactor, enough success has been made in developing uranium enrichment technology to provide nuclear fuel to allow the experimental procedure."

Building a light-water reactor, ostensibly for civilian energy purposes, would give the North Koreans a premise for enriching uranium. Uranium enriched to low levels is used in power reactors; left spinning, centrifuges will enrich uranium to the high levels needed for bombs.

Suspicions about a North Korean uranium enrichment program date back years.

North Korea worked with A.Q. Khan, creator of Pakistan's atomic bomb, to obtain the centrifuges needed for uranium enrichment before his operation was disrupted in 2003, former Pakistani President Pervez Musharraf said.