Enriched uranium

With the abandonment of its plutonium program, North Korea began an enriched uranium program. Pakistan, through Abdul Qadeer Khan, supplied key technology and information to North Korea in exchange for missile technology around 1997, according to U.S. intelligence officials.

This program was publicized in October 2002 when the United States asked North Korean officials about the program, [4]. Although the Agreed Framework specifically prohibited then-existing plutonium programs, not uranium, the U.S. argued North Korea violated the "spirit" of the agreement. In December 2002, the United States terminated the 1994 Agreed Framework, suspending fuel oil shipments.

North Korea responded by announcing plans to reactivate a dormant nuclear fuel processing program and power plant north of Pyongyang. North Korea soon thereafter expelled United Nations inspectors and withdrew from the Non-Proliferation Treaty.

Source : http://en.wikipedia.org