SEOUL, South Korea -- North Korea apparently conducted a widely anticipated nuclear test Tuesday, strongly indicated by an "explosion-like" earthquake that monitoring agencies around the globe said appeared to be unnatural.
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 toward 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 Defense 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 U.N. 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 U.N.
A world nuclear test monitoring organization 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 Tóth, the executive secretary of the Comprehensive Nuclear Test Ban Treaty Organization. The DPRK refers to North Korea's formal name, the Democratic People's Republic of Korea.
Kim Min-seok, a South Korean Defense Ministry spokesman, told reporters that 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 U.S. 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. Mr. Chi said a magnitude 3.9 magnitude earthquake and a magnitude 4.5 earthquake were detected in the North's 2006 and 2009 nuclear tests.
South Korean, U.S. and Japanese seismic monitoring agencies put the magnitude of Tuesday's quake between 4.9 and 5.2.
The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over the December rocket launch.
North Korea's politburo vowed to continue firing "powerful long-range rockets," but a statement by state media Tuesday made no mention of a nuclear test.