North Korea has apparently conducted a widely-anticipated nuclear test, strongly indicated by an "explosion-like" earthquake that appeared to be unnatural, world monitoring agencies said.
There was no confirmation from Pyongyang that it had conducted a test, which it has been threatening for weeks. Any test would be seen as another big step towards North Korea's goal of building a warhead that can be mounted on a missile.
It would also be a bold signal from young leader Kim Jong Un, who took power in December 2011 following the death of his father, Kim Jong Il.
The South Korean Defence Ministry, which raised its military alert level after the quake, said it was trying to determine whether it was a test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.
A nuclear test would also be a challenge to the United Nations Security Council, which recently punished Pyongyang for launching a long-range rocket seen as a covert test of ballistic missile technology.
In condemning that December rocket launch and imposing more sanctions on Pyongyang, the council had demanded a stop to future launches and ordered North Korea to respect a ban on nuclear activity - or face "significant action" by the UN.
A world nuclear test monitoring organisation said it detected what it called 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," said Tibor Toth, executive secretary of the Comprehensive Nuclear Test Ban Treaty Organisation. The DPRK refers to North Korea's formal name, the Democratic People's Republic of Korea.
"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," Mr Toth said.
Kim Min-seok, a South Korean Defence Ministry spokesman, said North Korea informed China and the United States of its plans to conduct a nuclear test. It was not clear when Pyongyang told Beijing and Washington.
The US Geological Survey as well as earthquake monitoring stations in South Korea detected an earthquake near a site where North Korea conducted its second nuclear test in 2009, according to the government-funded Korea Institute of Geoscience and Mineral Resources. "There is a high possibility that North Korea has conducted a nuclear test," said Chi Heoncheol, an earthquake specialist at the institute. He said a magnitude 3.9 and a magnitude 4.5 earthquake were detected in the North's 2006 and 2009 nuclear tests.