The United States and the international community are reacting with outrage and strongly condemning North Korea's latest nuclear test and the launch of three short-range missiles.
Speaking from the White House, U.S. President Barack Obama denounced North Korea's nuclear test and used blunt language to describe his concerns.
"North Korea's nuclear and ballistic missile programs pose a grave threat to the peace and security of the world and I strongly condemn their reckless action," said President Obama. "North Korea's actions endanger the people of Northeast Asia, they are a blatant violation of international law, and they contradict North Korea's own prior commitments."
President Obama says North Korea's actions have flown in the face of United Nations resolutions banning the country from developing weapons of mass destruction.
"North Korea will not find security and respect through threats and illegal weapons," said Mr. Obama. "We will work with our friends and our allies to stand up to this behavior and we will redouble our efforts toward a more robust international nonproliferation regime that all countries have responsibilities to meet."
Seismologists from around the world reported a tremor of about 4.5 magnitude in northeast North Korea, near where Pyongyang conducted its first test in 2006.
North Korean official media confirmed a test had taken place.
A North Korean newsreader says the country "successfully conducted one more underground nuclear test," demonstrating what Pyongyang calls its "self-defensive nuclear deterrent" to the entire world.
Russian and South Korean military officials say the nuclear explosion had a force of up to 20 kilotons, the same size as the bomb the U.S. dropped on Nagasaki, Japan in 1945.
South Korean officials say North Korea test-fired three short-range missiles shortly after the nuclear test.
The U.S. Chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, says he sees a growing belligerence on the part of North Korea and a rising defiance of international law.
"All of those things point to a country that I think continues to destabilize that region and actually in the long term, should they continue on to develop a nuclear weapons program, pose a significant threat to the United States," said Admiral Mullen.
America's top military officer says the United States has the ability to deal with any danger from North Korea.
"I am very confident that we can deal with the threat posed by North Korea," he said. "And it is not just the United States because there are many other countries, and certainly those in the region, who are equally concerned."
Worldwide, governments condemned North Korea's actions.
U.N. Secretary General Ban Ki-moon says the test is a clear and grave violation of Security Council resolutions.
U.S. Secretary of State Hillary Clinton spoke with the foreign ministers from South Korea and Japan. A State Department statement says Clinton stressed the importance of a strong and unified approach to a "threat to international peace and security."
South Korean officials say the test is a serious threat to peace and stability in the region.
The Russian Foreign Ministry says the six-party talks on North Korea are the only solution to the crisis.
The Chinese government says it is resolutely opposed to its communist neighbor and ally's actions.
Speaking while on a trip to Beijing, John Kerry, the chairman of the U.S. Senate Foreign Relations committee, says North Korea is trying to attract international attention.
He says this latest action is likely to hurt efforts to restart negotiations with Pyongyang.
"I do not think you reward it [nuclear test] by immediately going back to the table and having talks," said Senator Kerry. "I think North Korea is going to have to indicate some better willingness of how it intends to proceed in order to get immediately back to talks."
Some analysts say North Korea may be trying to gain leverage for negotiations with the international community.
They say the test may be part of an internal struggle. North Korean leader Kim Jong Il, who is believed to have suffered a stroke last year, has not named a successor.