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As retaliation against tighter U.N. sanctions, on Saturday North Korea defiantly threatened to expand its nuclear arsenal and begin a program of uranium enrichment--a threat it first made in response to U.N. condemnation of its early April rocket launch. Compared to North Korea's well-known plutonium production program, the nature of Pyongyang's highly enriched uranium (HEU) program is less clear.

The HEU issue first surfaced in October 2002, when North Korean First Deputy Foreign Minister Kang Sok Ju reportedly acknowledged the existence of such a program in response to allegations made by U.S. officials who were visiting Pyongyang. Ever since, though, Kang and other North Korean officials have consistently denied that the country is enriching uranium.