SEOUL, Sept. 4 -- North Korea announced early Friday that it is in the "final stage" of enriching uranium, a process that, if completed, would give it a second means of making a nuclear bomb.
In rejection of U.N. efforts to sanction its nuclear weapons program, the North's official Korean Central News Agency declared, "We've successfully done the experiment for enrichment of uranium and it has entered the final stage."
North Korea has twice tested nuclear devices that use plutonium, which is manufactured by a chemical process in a nuclear reactor. The last test, in late May, triggered international outrage and tough new U.N. sanctions.
As part of those sanctions, a North Korean arms shipment was seized last month off the United Arab Emirates. North Korea said Friday that it "will neither accept nor be binded" by the sanctions.
Enriched uranium offers a different way to make bombs. It uses centrifuges to spin hot uranium gas into weapons-grade fuel. North Korea announced in June -- after nearly seven years of denial -- that it had a program for making nuclear weapons from enriched uranium.
Outside experts have said that it would probably take the North several years to develop the uranium route to a bomb because the country lacks centrifuge materials, technology and know-how.
Iran, however, has mastered much of this technology and could help North Korea move forward, according to Siegfried S. Hecker, a periodic visitor to North Korea's plutonium complex at Yongbyon who was director of the Los Alamos National Laboratory.
North Korea and Iran have shared long-range missile technology that could enable both countries to deliver a nuclear warhead.
In its announcement Friday, the North also said that spent fuel rods from Yongbyon are being reprocessed and turned into weapons-grade plutonium. The reactor was partially disabled last year as part of a denuclearization deal with the United States and four other countries that the North subsequently canceled.
The United States and South Korea reacted negatively, with Washington's special envoy on North Korea saying any activities in the area of nuclear development are "of concern to us."
"These are issues we are dealing with as they arise and we maintain the need for cooperation and dialogue and complete denuclearization" of the Korean peninsula, Stephen Bosworth told reporters in Beijing during an Asia trip to discuss how to bring North Korea back to disarmament talks.
In Seoul, the South Korean foreign ministry said the North's behavior is "not tolerable."
"The government will deal sternly and consistently with North Korea's threats and provocations," foreign ministry spokesman Moon Tae-young said in a statement.
The North is thought to have enough plutonium to make six to eight bombs. In an interview this year, Hecker, co-director of Stanford University's Center for International Security and Cooperation, said there is enough plutonium in the spent rods for "one or two more" nuclear tests.
The statement from Pyongyang on Friday was part of a letter that the North Korean permanent representative has sent to the United Nations. It responds to questions from the U.N. sanctions committee about the North Korean arms shipment seized near the United Arab Emirates. The committee also asked Iran about the shipment.
In the first seizure of a North Korean weapons shipment since the May nuclear test, the United Arab Emirates inspected and held the Bahamas-flagged ship in early August. Diplomats have told news agencies that it contained rocket-propelled grenades and other arms.
"We don't feel the need to respond to the questions from the so-called sanctions committee," the North's state news agency quoted the letter as saying.
North Korea said Friday that it would not have tested a second nuclear device if the United Nations had not condemned its long-range missile launch in April.
Pyongyang said that it remains committed to removing nuclear weapons from the Korean Peninsula but that it will not participate in six-party talks, which include the United States, China, Russia, Japan and South Korea.
Those talks, Pyongyang said, are "used for violating [and] abusing North Korea's sovereign right and peaceful right to development."
North Korea is seeking bilateral talks with the United States, which the Obama administration has often said it is open to, but only after a resumption of the six-party talks.
In recent weeks, North Korea has appeared to be trying to calm tensions in Northeast Asia. It released several foreign nationals it had been holding, including two U.S. journalists. It also allowed the resumption of family visits between North and South Korea, reopened a border crossing and sent a delegation to attend the funeral of former South Korean president Kim Dae-jung.