President Obama called North Korea's latest nuclear test "a highly provocative act" that undermines regional stability and threatens international peace.
North Korea announced earlier today that it successfully tested a miniaturized nuclear device underground, according to state media.
Official state media said the test was conducted in a safe manner and is aimed at coping with "outrageous" U.S. hostility that "violently" undermines the North's peaceful, sovereign rights to launch satellites. Unlike previous tests, North Korea used a powerful explosive nuclear bomb that is smaller and lighter, state media reported.
North Korea kept up its belligerent talk today, vowing more provocative actions.
If the U.S. "makes this situation complex in hostility, we will come up with second and third reactions," North Korea said in a statement.
The regime blamed the nuclear test on the United States.
"Originally we didn't have to test the nuclear bomb and we didn't plan to," the statement said. "The major purpose of this test is to show our anger to the hostile actions of the U.S. and show off our will and capability to protect our sovereign right to the end."
The statement lashed out at the U.S.-led sanctions as well as the search of North Korean ships that are suspected of exporting illegal arms.
"We will consider the search of our ships and other sanctions by the international community as an act of war. And it will bring on our relentless retaliatory attack," North Korea said.
Obama also talked tough today.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community. The United States will also continue to take steps necessary to defend ourselves and our allies.
"The United States remains vigilant in the face of North Korean provocations and steadfast in our defense commitments to allies in the region," he added.
The U.N. Security Council will hold an emergency meeting on North Korea's nuclear test later this morning.
China's Ministry of Foreign Affairs released a statement expressing "firm opposition" to the test.
"We strongly urge the DPRK (North Korea) to abide by its denuclearization commitments, and to refrain from further actions that could lead to a deterioration of the situation," the statement read. "Safeguarding Korean Peninsula and East Asian peace and stability serves the shared interests of all parties."
China, North Korea's main ally in the region, has warned North Korea it would cut back severely needed food assistance if it carried out a test. Each year China donates approximately half of the food North Korea lacks to feed its people and half of all oil the country consumes.
Suspicions were aroused when the U.S. Geological Survey said it had detected a magnitude 4.9 earthquake Tuesday in North Korea.
The Comprehensive Test Ban Treaty Organization told ABC News, "We confirm that a suspicious seismic event has taken place 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 organization.
"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," Toth said in a statement on the organization's web site.
Kim Min-seok, a South Korean Defense Ministry spokesman, told reporters that North Korea informed United States and China that it intended to carry out another nuclear test, according to the AP. But U.S. officials did not respond to calls from ABC News Monday night.
The seismic force measured 6 to 7 kilotons, according to South Korea.
"Now that's an absolutely huge explosion by conventional terms. It's a smallish, but not tiny explosion by nuclear terms. It's about two-thirds the size of the bomb that the U.S. dropped on Hiroshima," James Acton, a senior associate at the Carnegie Endowment for International Peace, told ABC News.
North Korea threatened in January to carry out a "higher-level" test following the successful Dec. 12 launch of a long range rocket. At the time, North Korea's leader Kim Jong-Un said his country's weapons tests were specifically targeting the United States.