North Korean officials have this month taken foreign nuclear experts to what they said was a uranium enrichment facility at the country's main nuclear complex.
A uranium enrichment program would give Pyongyang a second way to obtain fissile material for making atomic bombs.
Washington has believed since 2002 that Pyongyang has such a program but the apparent sophistication of its effort could ignite fresh debate over how to deal with North Korea's unpredictable leadership and whether to resume talks aimed at ending its nuclear ambitions.
A senior Obama administration official would not say whether what the experts saw during their visits confirmed the existence of a uranium enrichment program.
Following is a look at the North's nuclear arms program:
THE YONGBYON FACILITIES
The Yongbyon complex is at the heart of the North's plutonium weapons program. It consists of a five-megawatt reactor, whose construction began in 1980, a fuel fabrication facility and a plutonium reprocessing plant, where weapons-grade material is extracted from spent fuel rods.
The site, about 100 km (60 miles) north of Pyongyang, also contains a 50-megawatt reactor whose construction was suspended under a 1994 nuclear deal with the United States. The reactor is nowhere near completion.
When fully operational, Yongbyon can produce enough fissile material for one nuclear bomb a year, experts say. Yongbyon was being taken apart under a disarmament-for-aid deal, but last year, in the face of U.S. hostility, the North said it was restoring parts of the plant. Satellite images taken show increased activity at the complex, however there have been no signs the North has restarted the reactor.
The North has also said it is building an experimental light-water reactor at the site. The North has tried to secure a light-water reactor for a number of years, claiming such a project would be for peaceful energy purposes. The type of reactor is considered relatively proliferation-resistant, meaning it is unlikely to be diverted for an arms program.
North Korea last year said it was enriching uranium, giving it another path for making atomic weapons. Uranium enrichment can be conducted away from the prying eyes of satellites and the North can fuel it with its ample supplies of natural uranium.
Experts said this month North Korean officials had shown them what they said was a uranium enrichment facility. The experts said they saw hundreds of centrifuges. The North Korean officials said they had 2,000 centrifuges, which are used in uranium enrichment, in operation but foreign experts who visited the facility were unable to verify that.
The foreign experts said they were not able to establish whether the plant was designed to produce only low-enriched uranium needed to make fuel for a power plant or the highly enriched uranium needed for bombs.
It is easier to design a nuclear bomb with highly enriched uranium (HEU) than plutonium but harder to make a nuclear warhead with HEU to mount on a missile.
The North's first test in October 2006 produced a relatively low yield in its explosive force, indicating problems with the bomb design or plutonium at its core, experts said.
A second test, in 2009, was stronger, but experts believe it may have only been about one-fifth to one-fourth as powerful as the plutonium bomb the U.S. dropped on Nagasaki in 1945.
U.S. officials said prior to the North's 2009 test it had produced about 50 kg (110 lb) of plutonium, which proliferation experts said would be enough for six to eight nuclear weapons. The North has since said it extracted more fissile material from spent fuel rods. This could be enough for one more bomb.
Even though it has exploded nuclear devices, North Korea has not shown it has a working nuclear bomb.
Experts say they do not believe the North can miniaturize an atomic weapon to place on a missile, but it is trying to develop such a warhead. It needs more nuclear testing to build one.
North Korea's aging fleet of Soviet-era bombers would also have difficulty evading the advanced air forces of regional powers to deliver a nuclear bomb outside the country.
(Reporting by Jeremy Laurence; Editing by Miral Fahmy)