Pyongyang's Korean Central News Agency also raised the specter of an expanding plutonium program, saying that "reprocessing of spent fuel rods is in its final phase and extracted plutonium being weaponized."
The North's Korean Central News Agency, reporting the letter, said North Korea was ready for "dialogue and sanctions" but that the North had "no choice but to take yet stronger self-defensive countermeasures."
The need for self-defense has been the North's rationale for its nuclear program through negotiations beginning in the early 1990s.
These talks culminated in the 1994 Geneva framework agreement under which North Korea shut down its five-megawatt reactor at Yongbyon and placed the complex under the eyes of inspectors from the International Atomic Energy Agency in return for the promise of construction of twin light water nuclear energy reactors to help meet the needs for power for the North's failing economy.
The Geneva framework fell apart in October 2002, however, after North Korea's vice foreign minister, Kang Sok Ju, acknowledged to James Kelly, then the US nuclear negotiator, that North Korea had a separate highly enriched uranium program. North Korea publicly denied the existence of the uranium program until earlier this year.
Both the United States and North Korea appear to be mingling diplomatic gestures with toughness while Mr. Bosworth visits first Beijing and then Seoul.