North Korea conducted similar nuclear tests in 2006 and 2009. Pyongyang has been threatening a 'higher-level' nuclear test for weeks despite warnings of severe repercussions from the UN Security Council.

“Reports indicate that Democratic People’s Republic of Korea (DPRK) has conducted a nuclear test on 12 February 2013. It is a matter of deep concern that DPRK has acted in violation of its international commitments in this regard. We call upon DPRK to refrain from such actions which adversely impact on peace and stability in the region,” said the Official Spokesperson in the Indian External Affairs Ministry, Syed Akbaruddin.

North Korea’s ally and neighbor China also condemned the incident and called for denuclearisation of the Korean Peninsula, preventing nuclear proliferation, and safeguarding peace and stability in Northeast Asia.

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

'We strongly urge (North Korea) to honour its commitment to denuclearisation and refrain from any move that may further worsen the situation...,' the statement said.

It further said: '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.'

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.

The North Korea’s third nuclear test caused tremors with the international monitoring agencies reporting a seismic event near the North Korea's Kilju county, North Hamkyung province.

The tremor, whose magnitude was estimated at between 4.9 and 5.1, was detected at 11:57am (0827 IST) with the epicentre in Kilju county, where the North Korea's Punggye-ri test site is located.

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

The US Geological Survey noted the seismic activity - also confirmed by monitors in South Korea, China and Japan - had a very shallow depth of just one kilometre (0.6 miles).

The South Korean defence ministry and the presidential Blue House both said they were trying to verify whether a nuclear detonation had taken place.

Pyongyang's promise of a 'higher-level' test had fuelled speculation it would be of a uranium device, compared to the plutonium ones detonated in 2006 and 2009.

A uranium test would confirm suspicions that the North has been secretly enriching weapons-grade uranium for years and open a path for Pyongyang to significantly expand its small nuclear arsenal.

Some experts had suggested a simultaneous test of both a plutonium and a uranium device.

Even with sophisticated seismic monitoring and 'sniffer' planes capable of detecting radioactive fallout, external analysis will provide only limited information on the test, especially if it was well-contained.

There will be particular concern at any sign that the North has made progress in the technically complex process of 'miniaturising' a bomb to fit on the head of a long-range missile.

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.

US President Obama said: '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 September 19, 2005, Joint Statement of the Six-Party Talks on North Korea's nuclear programmes and 'increases the risk of proliferation'.

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

North Korea 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.

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 said after an emergency security meeting presided over by President Lee Myung-bak.

A South Korean official said that North Korea had notified the US and China of its nuclear test plan a day earlier.

However, China said it firmly opposes the latest nuclear test conducted by North Korea.