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North Korea says it has “successfully“ carried out its third underground nuclear test, defying UN Security Council orders to shut down atomic activity.

Tuesday's explosion could take North Korea a big step closer to its goal of building a nuclear warhead small enough to be mounted on a long-range missile that could threaten the United States.

Official state media said the test was conducted in a safe manner and is aimed at coping with “outrageous” US hostility that “violently” undermines the North's peaceful, sovereign rights to launch satellites.

The North said it used a “lighter, miniaturised atomic bomb“ that had more explosive force than past tests.

“The high-level nuclear test, unlike in the past, had more explosive power and involved a miniaturised and lighter atomic bomb and was staged safely and perfectly,” the state-run Korean Central News Agency (KCNA) said.

The United States Geological Survey said that it had detected a 4.9-magnitude earthquake in North Korea.

South Korea moved quickly to condemn the test.

“This is an unacceptable threat to the security of the Korean peninsula and northeast Asia and a challenge to the whole international community,” presidential national security adviser Chun Young-Woo told reporters.

“The North will face grave responsibility for such provocations.”

UN leader Ban Ki-moon condemned the test as a “deeply destabilising” provocation.

“It is a clear and grave violation of the relevant Security Council resolutions,” Ban said of the blast, in comments given through his spokesman Martin Nesirky.

British Foreign Secretary William Hague labelled the test a “violation of United Nations Security Council resolutions”.

Hague said: “North Korea's development of its nuclear and ballistic missile capabilities poses a threat to international and regional security. Its repeated provocations only serve to increase regional tension, and hinder the prospects for lasting peace on the Korean peninsula.”

The nuclear test is North Korea's first since leader Kim Jong-Un took power in December 2011 following the death of his father, Kim Jong-Il, and marks a bold statement for the young leader as he unveils his domestic and foreign policy for a country long estranged from the West.

Experts say regular tests are needed to perfect North Korea's goal of building nuclear warheads small enough to be placed on long-range missiles.

This atomic test - North Korea's third since 2006 - is expected to take Pyongyang closer to possessing nuclear-tipped missiles designed to strike the US.

South Korea's Defence Ministry estimated the yield of the weapon to be 6-7 kilotons, Yonhap News Agency reported.

The UN Security Council was to meet on Tuesday (Wednesday AEDT) following a request from Seoul.

President Lee Myung Bak ordered a meeting of South Korea's security council in response to the test.

Japan was also expecting a stern response from the Security Council, chief cabinet secretary Yoshihide Suga said.

The United States told Japan on Monday that Pyongyang would conduct a nuclear test in the near term, he added.

North Korea tested atomic weapons in 2006 and 2009.

A third test had been expected. The secretive state in January said that it planned a further atomic and rocket tests in protest at expanded United Nations sanctions.

It did not specify when any tests would take place.

The UN sanctions were imposed following the test launch of a rocket by the North in December.