North Korea's blast was similar to previous nuclear tests, the Vienna-based monitoring agency CTBTO said early Tuesday, condemning it as a potential threat to international security if confirmed as a nuclear test.
North Korea has since confirmed that it had carried out what it called a successful underground nuclear test.
"The event shows explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK (Democratic People's Republic of Korea) nuclear tests," agency chief Tibor Toth said in a statement.
He did not however elaborate on the size of the blast, which South Korea's defence ministry said had an explosive yield of between six and seven kilotons.
"For now, further data and analysis are necessary to establish what kind of event this is," Toth said.
"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 had already detected what it called seismic events when North Korea carried out its two nuclear tests on October 9, 2006 and May 25, 2009.
The Comprehensive Nuclear Test-Ban Treaty Organisation is a key authority for providing data on detected nuclear events.
It has a network of about 340 monitoring stations around the world on the look-out for any signs -- both seismic and atmospheric -- of nuclear explosions.
However, it does not identify whether seismic events it picks up are a nuclear explosion, earthquake or chemical blast.
North Korea vowed last month to conduct a "high-level" nuclear test, its third, in retaliation for tightened UN sanctions following its December rocket launch.
It is not a signatory of CTBTO, which was adopted in 1996 by the United Nations General Assembly and ratified by 159 states.