Pyongyang/Seoul/Washington, Feb 13 (IANS): Highly secretive North Korea revealed Tuesday it had conducted a third nuclear test, drawing condemnation from around the world and prompting South Korea to put its military on high alert and summon a meeting of the UN Security Council.

Pyongyang, which conducted similar nuclear tests in 2006 and 2009, said the latest test, carried out in the Kilju county in North Hamkyung province, was to safeguard its national security against "hostile" American policy.

The world learnt about the nuclear test when an artificial earthquake of magnitude 4.9 was detected from the location where the North Korean nuclear test facility is located.

North Korea claimed the nuclear test was successful, Xinhua reported, citing the country's official KCNA news agency.

US President Barack Obama slammed the "highly provocative act".

"This is a highly provocative act that, following its December 12 ballistic missile launch, undermines regional stability, violates North Korea's obligations under numerous United Nations Security Council resolutions."

He said North Korea's latest move contravenes its commitments under the Sep 19, 2005, Joint Statement of the Six-Party Talks on North Korea's nuclear programmes and "increases the risk of proliferation".

Calling North Korea's nuclear weapons and ballistic missile programmes as a threat to US national security and to international peace and security, Obama said the US "remains vigilant in the face of North Korean provocations and steadfast in our defense commitments to allies in the region".

Pyongyang vowed to proceed with missile and nuclear tests targeting the US, its "sworn enemy", after the UN Security Council adopted a resolution to condemn its Dec 12 rocket launch.

South Korea's military was immediately put on heightened alert to deter potential cross-border provocations, while the South Korean and US forces upgraded their border surveillance level.

Japanese Prime Minister Shinzo Abe said the nuclear test conducted by North Korea was "regrettable".

The South Korean government called it a "clear violation" of UN resolutions.

"North Korea will not be able to avoid being held accountable for (the test)," National Security Adviser Chun Yung-Woo told reporters after an emergency security meeting presided over by President Lee Myung-bak.

Neighbour China expressed its "firm" opposition to the latest nuclear test.

North Korea "conducted another nuclear test in disregard of the common opposition of the international community", said the Chinese foreign ministry, adding that "the Chinese government is firmly opposed to this act".

The foreign ministry said it was the firm stand of China to bring about denuclearisation of the Korean peninsula, prevent nuclear proliferation, and safeguard peace and stability in northeast Asia.

"The Chinese government calls on all parties to respond in a cool-headed manner and persist in resolving the issue of denuclearisation of the Peninsula through dialogue and consultation within the context of the Six-Party Talks," it said.

The six-party talks, a negotiation mechanism that includes North Korea, South Korea, the US, China, Japan and Russia, were launched in 2003 but stalled in December 2008. North Korea quit the talks in April 2009.

NATO and European Union (EU) also condemned the nuclear test.

"We condemn in the strongest terms the test by North Korea of a nuclear weapon, conducted in flagrant violation of relevant United Nations Security Council resolutions," said NATO.

Condemning the test "in the strongest possible terms", the EU foreign policy chief Catherine Ashton said it was "one more step in a long-running programme to develop a nuclear weapon capability".

India described the nuclear test as a "matter of deep concern" and called upon Pyongyang to "refrain" from actions that would adversely affect peace and security in the region.

"It is a matter of deep concern" that North Korea carried out the nuclear test in violation of UN Security Council resolutions, said the Indian external affairs ministry's spokesperson.

"We call upon DPRK to refrain from such actions that can have an adverse effect on the peace and stability of the region," he added.

Pakistan also expressed regret over Pyongyang's nuclear test.

Meanwhile, the UN Security Council also met behind closed doors Tuesday in an emergency session on the latest nuclear test by North Korea. The meeting was called by South Korea, which holds the rotating council presidency for February.