North Korea announced Friday that its process of enriching uranium is nearly complete, giving it a new way to make nuclear bombs as the U.S. and regional powers discuss how to bring the communist country back to disarmament talks.
The move raises concerns that North Korea may soon produce uranium-based bombs in addition to those made from plutonium.
The U.S., China, Japan, Russia and South Korea had been trying for years to persuade North Korea to dismantle its plutonium-based nuclear program, which experts say has yielded enough weaponized plutonium for at least half a dozen bombs, in exchange for much-needed aid.
After hashing out a 2007 disarmament-for-aid deal, North Korea walked away from those talks earlier this year in anger over the rising international outcry over a rocket launch widely condemned as a disguised test of its long-range missile technology.
Uranium can be enriched in relatively inconspicuous, underground factories, and could provide North Korea with an easier way to build nuclear bombs, according to experts in the U.S. and at South Korea's Institute of Nuclear Nonproliferation and Control.
Uranium-based bombs may also work without requiring test explosions like the two carried out by North Korea this May and in 2006 for plutonium-based weapons.
Washington's special envoy on North Korea, Stephen Bosworth, said any nuclear development in North Korea was a matter of concern.
"We confirm the necessity to maintain a coordinated position and the need for a complete denuclearization of the Korean peninsula," he said in Beijing during an Asian trip to discuss North Korea with counterparts in China, South Korea and Japan.
The U.S. had long suspected that the North also had a covert uranium enrichment program, which would give it a second source of nuclear material. North Korea for years denied the claim but revealed in June that it was prepared to start enriching uranium.
"Experimental uranium enrichment has successfully been conducted to enter into completion phase," North Korea said in a letter to the U.N. Security Council carried Friday by its official Korean Central News Agency.
Verifying North Korea's claim on uranium enrichment won't be easy, South Korean Defense Ministry spokesman Won Tae-jae said, adding that it could be a negotiating tactic.
However, the announcement suggests the regime has made progress in research and development in its uranium program in a small pilot factory, said Lee Choon-geun of South Korea's state-funded Science and Technology Policy Institute. Still, he said it could take at least five years to build a uranium-based bomb.
North Korea also said Friday it is continuing to weaponize plutonium. The 2007 deal had called for disabling its reactor.
The tough talk indicated North Korea's impatience with the U.S. as Washington continues to pursue sanctions against the North despite a series of overtures from it in recent weeks.
"We are prepared for both dialogue and sanctions," the North said in the letter to the Security Council. If some permanent council members "wish to put sanctions first before dialogue, we would respond with bolstering our nuclear deterrence first before we meet them in a dialogue," it said.
A Security Council resolution sought to punish North Korea for carrying out an underground nuclear test in May by tightening an arms embargo and authorizing ship searches on the high seas to try to rein in the North's nuclear program. It also ordered an asset freeze and travel ban on companies and individuals involved in the its nuclear and weapons programs.
North Korea said it does not oppose the denuclearization of the Korean peninsula, but warned it would be left with no choice but to take "yet stronger self-defensive countermeasures" if the standoff continues. It did not elaborate on the possible countermeasures.
Bosworth, who is to meet with South Korean officials starting Saturday, reaffirmed Washington's position to engage North Korea — but only within the framework of the six-nation disarmament talks.
The North has long sought one-on-one negotiations with Washington, claiming the nuclear dispute is an issue between Pyongyang and Washington.
It says it needs the nuclear program as a security guarantee against a threat from the U.S., which has 28,500 troops based in South Korea, which technically remains at war with the North because their three-year conflict ended in 1953 with a truce, not a peace treaty.
Friday's action is a message from North Korea to Washington seeking "direct dialogue while warning that its nuclear stockpile will increase unless Washington sits down with Pyongyang for negotiations," said Lee Sang-hyun, an analyst at the private Sejong Institute think tank near Seoul.
South Korea expressed regret over the North's announcement.
"The North's move to continue provocative steps ... can never be tolerated. We will deal with North Korea's threats and provocative acts in a stern and consistent manner," the Foreign Ministry said in a statement.
Japan urged North Korea to refrain from actions and remarks that could heighten tensions.
"We will definitely not tolerate North Korea possessing nuclear weapons," Japanese Foreign Minister Hirofumi Nakasone said.