"For now, further data and analysis are necessary to establish what kind of event this is," he added.
"If confirmed as a nuclear test, this act would constitute a clear threat to international peace and security, and challenges efforts made to strengthen global nuclear disarmament and non-proliferation, in particular by ending nuclear testing."
The agency earlier announced that its monitoring stations had picked up "evidence of an unusual seismic event in the Democratic People's Republic of Korea," following reports from China and Japan about a suspected nuclear test.
The CTBTO's had already detected what it called seismic events when North Korea carried out its two nuclear tests on 9 October 2006 and 25 May 2009.
The Comprehensive Test Ban Treaty Organisation is a key authority for providing data on detected nuclear events, but does not identify whether seismic events it picks up are a nuclear explosion, earthquake or chemical blast.