(RFE/RL) -- North Korea has announced that it is in the final phase of enriching uranium, a process that would give it a second method of making nuclear weapons, in addition to its known plutonium-based program.
Pyongyang also announced that it is reprocessing plutonium from spent nuclear fuel rods in order to build atomic weapons.
The announcements came in a letter from North Korean diplomats informing the UN Security Council that Pyongyang is forging ahead with its nuclear programs despite international calls to abandon them.
Quoted by North Korean state media, the letter says the development is a response to the tightening of UN sanctions against North Korea over recent nuclear and rocket tests.
North Korea has tested two plutonium-based nuclear devices -- the last one in May -- and had been suspected of running a secret program to enrich uranium for atomic weapons.
The North also carried out a failed rocket test in April, which it said was an attempt to launch a satellite.
U.S. President Barack Obama said the rocket could be used as a long-range missile and that the test was a "provocation" underscoring the need for coordinated international action to prevent the spread of nuclear weapons.
Nuclear experts say a switch from plutonium to uranium by North Korea is alarming because uranium enrichment programs can be operated in relatively small laboratories, making it difficult for them to be detected by U.S. spy satellites. They say it also could lead to enhanced cooperation between North Korea and Iran or other states trying to start their own nuclear weapons programs.
South Korean Defense Ministry spokesman Won Tae-jae says it will not be easy to verify North Korea's claims on uranium enrichment. But he said it is possible the announcement could be a bluff meant as a negotiating tactic.
But South Korean Foreign Ministry spokesman Moon Tae-young responded angrily to the North Korean announcement.
"It cannot be tolerated that North Korea discounted the international community's decisive call for the denuclearization of North Korea and regional peace and stability in Northeast Asia," he said.
Nevertheless, Moon says South Korea will continue to support international efforts for the denuclearization of the Korean peninsula.
"We will closely cooperate with related countries to urge North Korea to abolish its nuclear weapons program and return to the six-way talks as soon as possible for denuclearization in the true sense," he said.
North Korea had declared in mid-April that it would pull out of international talks for good and resume its nuclear enrichment program. North Korea also has expelled all international nuclear inspectors from the country.
'Maintain Coordinated Position'
Stephen Bosworth, the U.S. special envoy on North Korea, said in Beijing that any bilateral talks with North Korea on the issue must be part of the six-party process that includes the United States, China, Russia, and Japan, as well as North and South Korea.
"Obviously, anything that the North is doing in the area of nuclear development is of concern to us. And these are issues that we are dealing with as they arise," Bosworth said. "I think for all of us it reconfirms the necessity to maintain a coordinated position on the need for the complete, verifiable denuclearization of the Korean peninsula."
The U.S., China, Japan, Russia, and South Korea have been trying for years to persuade North Korea to dismantle its nuclear program in exchange for much-needed aid and other concessions.
The North says it needs the nuclear program as a security guarantee against a threat from the United States, which has nearly 30,000 troops based in South Korea.
North and South Korea technically have remained at war since their three-year conflict in the early 1950s was paused under the terms of a 1953 armistice. North Korea unilaterally withdrew from that armistice in May of this year.