North Korea said it had successfully conducted a test of a miniaturised hydrogen nuclear device, which would mark a significant advance in the isolated state's technological capability.
"This act constitutes a breach of the universally accepted norm against nuclear testing," Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO).
"It is also a grave threat to international peace and security."
Wednesday's detonation was North Korea's fourth nuclear test, and second since young leader Kim Jong Un came into power in 2011.
The isolated country last tested a nuclear device in 2013.
More than 160 countries have ratified the Comprehensive Nuclear Test Ban Treaty (CTBT) since 1996. India and Pakistan have also conducted nuclear tests since then and are among eight countries including the United States and China preventing the treaty coming into force.
"I sincerely hope that this will serve as the final wake-up call to the international community to outlaw all nuclear testing by bringing the CTBT into force," Zerbo said.