North Korea has confirmed that it has conducted a "safe and perfect" nuclear test defying international warnings.
The state-run KCNA news agency confirmed that the third nuclear test was "successful" and that it was conducted "in a safe and perfect manner".
The test used a miniaturised nuclear device with greater explosive force, added the official news agency.
Earlier, the US Geological Survey (USGS) 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 South Korea's meteorological agency suspected the quake was artificial, according to the Yonhap news agency. The epicentre of the quake is also likely to be closer to the North's reported nuclear test site.
South Korea confirmed that the North had conducted the test. "In close cooperation with the US, 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 Defence Ministry spokesperson Kim Min-seok.
A statement from Tibor Toth, executive secretary of the global agency on banning nuclear tests, CTBTO, said the organisation's monitoring stations had picked up evidence of an unusual seismic event in North Korea.
"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."
South Korea condemned the test, saying it threatens regional security. The country's security has also been beefed up in the wake of tensions arising from the test.
South Korea has called for an emergency meeting of the UN Security Council. The meeting is likely to take place later in the day.