May 25: United Nations Secretary-General Ban Ki-moon, left, called for a U.N. Security Council response to North Korea's latest nuclear test Monday, speaking from Copenhagen, Denmark.
May 24: A map of North Korea shows the epicenter of a 4.7 magnitude earthquake believed to have occurred as a result of the nuclear test.
File: The cooling tower of North Korea's Yongbyon nuclear complex, a plutonium-producing reactor, was demolished on June 27, 2008.
Apr. 7: In this image made from video, a rocket launch is seen in Musudan-ni, North Korea on April 5.
April 9: North Korean leader Kim Jong II attends the first session of Supreme People's Assembly of the country.
April 5: A South Korean soldier watches a TV news program on the North Korean rocket launch at a train station in Seoul.
The launchpad in North Korea which houses the three-stage Taepodong-2 missile.
Mar. 29: Japanese forces guard anti-missile interceptors deployed in Tokyo ahead of North Korea's rocket launch.
The U.N. chief said he strongly deplored a second nuclear test by North Korea that clearly violated a United Nations Security Council resolution, as the council called an emergency session Monday to discuss the matter.
The five permanent veto-wielding members of the council — the United States, Russia, China, Britain and France — met behind closed doors ahead of the meeting of the full 15-member council.
Japan, which called for the emergency meeting, said North Korea's "irresponsible" nuclear test and a missile launch in April had challenged the authority of the U.N.'s most powerful body "and the response must be firm."
"It's a very clear challenge," said Japan's U.N. Ambassador Yukio Takasu, a non-permanent council member. "So therefore we need a really, really clear and firm message from this — preferably a resolution."
Takasu refused to say whether Japan would seek new sanctions against North Korea, saying he wanted to consult with other council members. "The important thinking is a unified message from the council," he said.
North Korea claimed it carried out a powerful underground nuclear test Monday that was much larger than one it conducted in 2006. Russia's Defense Ministry confirmed an atomic explosion occurred early Monday in northeastern North Korea and estimated that its strength was similar to bombs that devastated Hiroshima and Nagasaki in World War II.
"I sincerely hope that the Security Council will take necessary corresponding measures," U.N. Secretary-General Ban Ki-moon told The Associated Press during a visit to Copenhagen, declining to specify what further measures, or sanctions, he would urge the council members to take.
Ban, who was in the Danish capital for a global business summit on climate change, said he would closely monitor the meeting in New York.
A statement issued by his spokeswoman later Monday said "the secretary-general strongly deplores the conduct of an underground nuclear test by the Democratic People's Republic of Korea, in clear and grave violation of the relevant Security Council resolutions."
New testing by North Korea would undermine peace and security in the region, Ban told AP, and he urged the communist nation "to refrain from taking any actions which will deteriorate the situation."
He urged the Security Council in the statement "to send out a strong and unified message, conducive to achieving the goal of denuclearization of the Korean peninsula and peace and security in the region."
The 2006 U.N. resolution, adopted after North Korea conducted its first nuclear test explosion in October of that year, banned the North from conducting further nuclear tests.
Ban also said the announcement from North Korea's official news agency that it carried out an underground nuclear test Monday "will create negative impact to ongoing negotiation on nuclear disarmament."
"They should have come to the dialogue table and resolved all the issues through peaceful means," he said.
Pyongyang also test-fired three short-range, ground-to-air missiles Monday from the same northeastern site where it launched a rocket last month, South Korea's Yonhap news agency reported, citing unnamed sources. U.N. Security Council resolutions bar North Korea from engaging in any ballistic missile-related activity.
North Korea's actions swiftly drew international condemnation.
President Barack Obama said the United States would work with allies around the world to "stand up to" North Korea. He said the latest nuclear tests "pose a grave threat to the peace and security of the world."
European Union Commission President Jose Manuel Barroso said he hoped "the international community will be very clear in its reaction. I also encourage the North to refrain from all kinds of provocation."
Even China joined the chorus of disapproval, saying it "resolutely opposed" the test.
The U.N. Security Council last month rebuked North Korea for the April 5 rocket liftoff, which many nations saw as a cover for testing its long-range missile technology.
In response, North Korea announced it was quitting disarmament talks and restarting its atomic facilities after the U.N. Security Council imposed sanctions on three major North Korean companies due to Pyongyang's April rocket launch. The six-party talks, which began in 2003, had involved North Korea, South Korea, Russia, China, Japan, and the United States.