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.
North Korea Warns of More 'Reactions' to U.S. Sanctions
The suspicious tremor comes just hours before President Obama is to give the State of the Union address, and it marks the first diplomatic test in the region for new Secretary of State John Kerry.
Also, South Korea's new president, Park Geun-hye, is scheduled to be sworn in Feb. 25. One of North Korea's biggest holidays, Kim Jong-il's birthday, falls on Feb. 16.
Both of North Korea's previous tests used a plutonium-based method for making bomb fuel. The first was deemed a failure, the second only slightly less so. If the most recent test used HEU (highly enriched uranium), a far more difficult-to-detect method of producing bomb fuel, it would be a significant and worrisome step forward for North Korea's weapons program.
U.S. officials and experts say they believe North Korea has enough plutonium for four to 10 devices. But the country's small nuclear reactor in Yongbyon was dismantled in 2008. It is believed North Korea's plutonium stockpile is therefore limited.
Both HEU and plutonium can be used to fuel a miniaturized nuclear device, or warhead. The North Koreans are not believed to currently have the technology to produce a warhead small enough to travel on the end of a long-range rocket. Observers estimate such capability is still many years away.
Plutonium is produced in an above-ground nuclear facility that is relatively simple to monitor using satellite imagery, a drawback for a regime intent on keeping its nuclear program shrouded in secrecy. By comparison, a highly enriched uranium facility, or centrifuge plant, is easy to hide underground. But plutonium bomb fuel is widely considered to be better suited to the kind of warhead North Korea hopes to develop than HEU.
But many believe that A.Q. Khan, the father of Pakistan's atomic bomb who has admitted to selling nuclear secrets to rogue states, sold the North Koreans a Pakastani HEU bomb design that could travel on one of the country's existing rockets.
As Siegfried Hecker, a former director of the Los Alamos National Laboratory who toured the Yongbyon site in 2010, wrote in February on the Web site of Foreign Policy magazine, "If Khan provided both design and test-performance data, Pyongyang may have decided that HEU, albeit less effective than plutonium, was a quicker and more certain route to miniaturized nuclear devices."
Said retired Marine Col. Stephen Ganyard, a former deputy assistant secretary of state for plans, programs and operations in the State Department's Bureau of Political-Military Affairs: "A successful test gives the North Koreans credibility with their main clients, Iran, and ensures a continued flow of Iranian funds to support further development," s
ABC News' Joohee Cho contributed to this report.