The Democratic People's Republic of Korea (DPRK) said it successfully conducted a nuclear test on Monday, arousing immediate concerns and various responses from the world community.
UN Secretary General Ban Ki-moon said in Copenhagen that he was "deeply disturbed" by the nuclear test, telling reporters that he was closely following the events.
The UN Security Council would hold an emergency meeting on Monday to discuss the nuclear test.
South Korea's Defense Ministry said Monday it launched a "crisis management team" of general-level officers.
"The team will come up with measures to respond to the nuclear test," said Lee Bung-woo, a spokesman for the Ministry of National Defense, adding the military is yet to put its troops on heightened alert.
Japanese Prime Minister Taro Aso said Monday that the DPRK nuclear test is a "grave challenge" to the Nuclear Non-Proliferation Treaty regime and a "clear violation" of a UN Security Council resolution banning the country from nuclear activity.
The Japanese government has set up a special task force at the emergency management center of Prime Minister Aso's office.
U.S. President Barack Obama condemned the test as a "threat to international peace and security."
The behavior increases tensions and undermines stability in Northeast Asia, said Obama in an immediate statement, adding that such provocations will only serve to deepen the DPRK's isolation.
Obama accused the DPRK of "recklessly challenging the international community" with its new underground nuclear test.
The Russian Foreign Ministry voiced concern about the test, saying that the nuclear test threatens regional stability, violates the will of the UN Security Council and is a blow to non-proliferation efforts.
"As a permanent member of the UN Security Council and a depositary state of the Treaty on the Nonproliferation of Nuclear Weapons (NPT), Russia condemns acts of this kind, which undermines the NPT regime," the Interfax news agency quoted Russian presidential spokeswoman Natalya Timakova as saying.
"Initiators of decisions on nuclear tests bear personal responsibility for them to the world community. It is absolutely clear that politics and diplomacy are the only way for the DPRK to achieve security for itself," she said.
The European Union (EU) also expressed concerns about the development.
EU foreign policy chief Javier Solana branded Monday the nuclear test as "irresponsible acts," and called for a tough response from the international community.
These irresponsible acts by the DPRK warrant a firm response by the international community, Solana said in a statement, adding the EU will be in contact with its partners to discuss appropriate measures.
The reported nuclear test is "very, very worrying," EU external relations commissioner Benita Ferrero-Waldner said Monday.
Australia's Prime Minister Kevin Rudd strongly condemned the DPRK's nuclear test, saying the move was provocative and will pose a threat to peace and security of the Asian Pacific region.
New Zealand's Foreign Minister Murray McCully said Monday in a statement that the DPRK's move was a provocative act that threatened the stability of the Korean Peninsula. New Zealand called on the DPRK to return to the six-party talks, said the statement.
New Zealand's Prime Minister John Key also expressed disappointment over the DPRK's nuclear test.
The foreign ministry of Singapore said it was concerned and disappointed over the DPRK's move, which was rude and dangerous and may lead to instability of the region.
Peaceful dialogues are the only way to ensure the DPRK's security and economic interests, the ministry said.
The German government called the test as an "irresponsible provocation" that threatens international stability.
British Prime Minister Gordon Brown denounced the nuclear test on Monday as a "danger to the world" that will undermine peace prospects on the Korean Peninsula.
The French government Monday called on Pyongyang to "refrain from any new provocation."
In a statement issued by the Foreign Ministry, the French government called on the DPRK "to resume discussions within the Group of Six with the aim of the complete, irreversible and verifiable dismantling of its nuclear program."
France "asks for the strongest sanctions" against the DPRK government, a French government spokesman was quoted as saying by French media Monday.
Switzerland's foreign ministry condemned the DPRK's move in a statement Monday, saying it breached relevant UN resolutions, threatened regional peace and security, and went against international efforts of nonproliferation of nuclear weapons.
The DPRK said on Monday it successfully conducted "one more" underground nuclear test earlier in the day.
In a statement released by the official news agency KCNA, the DPRK government said the test was "part of the measures to bolster up its nuclear deterrent for self-defense in every way as requested by its scientists and technicians."
But the brief statement did not give any details about the test, including its location.
The Seoul-based Korea Meteorological Administration said it detected an "artificial earthquake" of 4.5 magnitude around local time 09:54 a.m. (0054 GMT) Monday in North Hamkyong Province, 10 to 15 km away from the site where the DPRK staged its first nuclear test in 2006.
The test came after the UN Security Council adopted a presidential statement in late April, condemning the April 5 rocket launch by the DPRK and demanding the country "not conduct any further launch."
Pyongyang subsequently announced withdrawal from the six-party talks on nuclear disarmament and reactivation of nuclear facilities in protest of the UN statement.