The test has been condemned by South Korea, Japan, the United Kingdom and UN Chief Ban Ki-moon. South Korea’s Defense Ministry estimates the power of the presumed blast at 10 kilotons.

"The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK (North Korea) nuclear tests," the Comprehensive Nuclear Test Ban Treaty Organization’s Executive Secretary Tibor Toth said in a statement on Tuesday.

“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,” Toth added.

Seismic activity was detected in North Korea at 11:58 a.m. Korean time (02:58 GMT) on Tuesday, fueling suspicions that the country had carried out a nuclear test, as the area is not earthquake-prone.

The quake’s epicenter was just one kilometer deep, and was close to a known nuclear test site in North Korea.

North Korea notified the United States and China in advance that it would carry out a nuclear test, Yonhap reported.

The UN Security Council is to convene for an emergency meeting at 14:00 GMT.

North Korea vowed on January 23 to strengthen its military capabilities and step up its controversial nuclear program. The announcement came as an apparent response to the UN Security Council’s new sanctions on the country over December’s rocket launch.

Russian emergency service officials and forecasters have measured radiation levels in the Primorye Territory, which shares a border with North Korea, and reported that that “the daily average indicators have not changed.”