An official Web site run by North Korea and monitored by South Korea's Yonhap News Agency, carried the text of the reclusive nation's revised constitution that included the phrase "a nuclear-armed state."
"At first, nuclear-weapon state status is in line with the terms of the Nuclear Non-Proliferation Treaty, but North Korea itself has admitted that it is not a member of the NPT," South Korean Foreign Ministry spokesman Cho Byung-jae said.
North Korea abandoned the NPT in early 2003.
Cho called on North Korea to "implement its commitments and give up all nuclear weapons programs" under a 2005 agreement in which North Korea agreed to give up its nuclear programs in return for security guarantees and economic assistance from five other nations participating in the six-party talks. Pyongyang boycotted follow-up negotiations.
The revised constitution said North Korea's late leader, Kim Jong Il, "has turned our fatherland into an invincible state of political ideology, a nuclear-armed state and an indomitable military power, paving the ground for the construction of a strong and prosperous nation," Yonhap reported.
The revision was made by lawmakers in April.
Analysts in Seoul said the constitutional proclamation of a "nuclear-armed state" could cloud prospects of resuming the long-stalled six-party talks among the two Koreas, China, Japan, Russia and the United States.