Seoul has confirmed that North Korea has conducted a third nuclear test defying serious international warnings.
"In close cooperation with the U.S., South Korea has been closely watching the possibility of its third nuclear test, and based on our analysis of various materials, it is presumed that the North carried out a nuclear test," said the Defence Ministry spokesperson Kim Min-seok.
South has also condemned the test saying it threatens the regional security. Seoul's security measures have also been beefed up over the ongoing situation.
The US Geological Survey (USGS) has detected a quake of 4.9 magnitude in North Korea indicating the likelihood of a nuclear test.
The tremor occurred at 11:57 am local time (02:57 am GMT) while Seoul's meteorological agency suspects the quake as artificial, according to the Yonhap news agency. The epicentre of the quake is also likely to be closer to North's suspected nuclear test site.
South Korea has also called for an emergency meeting of the UN Security Council. The meeting is likely to take place later in the day.
"Today our monitoring stations picked up evidence of an unusual seismic event in the Democratic People's Republic of Korea (DPRK). The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK nuclear tests. For now, further data and analysis are necessary to establish what kind of event this is. 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," read a statement from Tibor Toth, executive secretary of the global agency on banning nuclear tests, CTBTO.
Seoul's defence ministry spokesman Kim Min-seok earlier said: "The military is closely watching the latest movement because North Korea could fire off (a long-range rocket) or something else while testing its nuclear device. South Korea and the US have mobilised all intelligence assets to observe attempts to (fire off a long-range rocket)."
Pyongyang had earlier fired several short-range missiles when it conducted the nuclear test in 2009.
The North has been sternly warned by several countries over its nuclear ambitions.