N Korea faces nuclear fallout

North Korea said the test was its "first response" to US threats and pledged "second and third measures of greater intensity" in the future.

State news agency KCNA said nuclear testing was necessary to defend the country's security and sovereignty.

The test took place in the country's north at a facility on Nuclear Test Road.

"The test was conducted in a safe and perfect way on a high level, with the use of a smaller and light A-bomb, unlike the previous ones, yet with great explosive power," KCNA said.

The UN security council unanimously condemned the test after an emergency meeting, saying it would work on "appropriate measures" to deal with North Korea's flouting of resolutions banning tests.

The US claims that Pyongyang is trying to develop the ability to fire nuclear missiles at the country.

President Barack Obama said the nuke tests do not make North Korea more secure, but instead it has "increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction."

However in Britain CND general secretary Kate Hudson said nuclear-armed states including the US shared the blame for Pyongyang's nuclear programme.

"As long as nuclear-armed states conclude that they are better off with these monstrous weapons, others will come to the same conclusion," she said.

"Let us condemn North Korea seeking nuclear weapons while we condemn nuclear-armed states for refusing to fulfil their legal and moral obligations to eliminate their arsenals."

China, which has led efforts for a peace on the Korean peninsular and is seen as an ally of North Korea, also said it "firmly opposed" the tests.

Chinese Foreign Minister Yang Jiechi summoned ambassador Ji Jae Ryong to lodge a solemn representation.

"We strongly urge the Democratic People's Republic of Korea to honour its commitment to denuclearisation and refrain from any move that may further worsen the situation," the Chinese ministry said in a statement.

Beijing also urged North Korea to re-engage with six-party talks on non-proliferation.