North Korea has confirmed it has carried out a third nuclear test after monitoring agencies reported an “unusual seismic event”.

North Korea has drawn widespread condemnation after conducting a nuclear test in defiance of international bans – a development signalled by an earthquake detected in the country and later confirmed by the regime.

The test, which took place in the north-east of the country just before noon local time, could bring North Korea a step closer to developing a nuclear warhead small enough to be mounted on a long-range missile and possibly bringing the west coast of the US within striking distance.

The authorities in Pyongyang said scientists had set off a “miniaturised” nuclear device with a greater explosive force than those used in two previous nuclear tests, in 2006 and 2009.

“It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturised and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment,” KCNA, the North’s official news agency, announced.

The agency said the test had been in response to “outrageous” US hostility that “violently” undermined the regime’s right to peacefully launch satellites – a reference to the condemnation and tighter sanctions that greeted Pyongyang’s successful rocket launch almost two months ago.

Tuesday’s test was quickly condemned by the UN general secretary, Ban Ki-moon, who said it was “deplorable” that Pyongyang had defied international calls to abandon it, adding that it was a “clear and grave violation of the relevant UN security council resolutions”.

Barack Obama said the test was a highly provocative act that violated security council resolutions and posed a threat to US and international security. The US president called for “further swift and credible action by the international community” against North Korea.

A 4.9 magnitude earthquake was detected just north of a site where Pyongyang conducted nuclear tests in 2009.

The South Korean defence ministry, which raised its military alert level after the quake, said the blast had an explosive yield of between six and seven kilotons (6-7kt) and was of “enormous destructive power”.

In comparison, the nuclear weapons detonated above Hiroshima and Nagasaki in 1945 carried a yield of around 20kt each.

A spokesperson for the UN boss Ban Ki-moon said: “The Secretary-General condemns the underground nuclear weapon test conducted by the Democratic People’s Republic of Korea today.

After the blast took place a North Korean newsreader urged the Korean People’s Army (KPA) to prepare for combat against its foes.

She stressed that If an order was given “the KPA should blow up the stronghold of aggression at a strike”.

“And wipe out the brigandish US imperialists and South Korea puppet army to the last man and thus accomplish the historic cause of national reunification.”

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