North Korea has said it would carry out further rocket launches and a nuclear test that would target the United States.

The announcement by the country's top military body came a day after the UN Security Council agreed a US-backed resolution to censure and sanction North Korea for a rocket launch in December that breached its bans.

North Korea is believed by South Korea and other observers to be "technically ready" for a third nuclear test.

The decision to go ahead rests with leader Kim Jong-un, who pressed ahead with the December rocket launch in defiance of the UN sanctions.

China, the one major diplomatic ally of the isolated North, agreed to the US-backed resolution and it also supported resolutions in 2006 and 2009 after Pyongyang's two earlier nuclear tests.

Today's statement by North Korea represents a huge challenge to Beijing as it undergoes a leadership transition with Xi Jinping due to take office in March.

North Korea has rejected proposals to restart the so-called six-party talks aimed at curbing its nuclear capacity.

Analysts said the North could test as early as February as South Korea prepares to install a new, untested president.

It is thought that it could choose to stage a nuclear explosion to coincide with former ruler Kim Jong-il's 16 February birthday.

The US has denounced the threats as "needlessly provocative".

White House spokesman Jay Carney told reporters that any such action would violate UN resolutions and would increase Pyongyang's diplomatic isolation.

The North was banned from developing missile and nuclear technology under sanctions dating from its 2006 and 2009 nuclear tests.

A South Korean military official said the concern now is that Pyongyang could undertake a third nuclear test using highly enriched uranium for the first time, opening a second path to a bomb.

North Korea's 2006 nuclear test using plutonium produced a puny yield equivalent to one kiloton of TNT.

13-18 kilotons was used for the Hiroshima bomb and US intelligence estimates put the 2009 test's yield at roughly two kilotons.

North Korea is estimated to have enough fissile material for about a dozen plutonium warheads.

Intelligence reports suggest that it has been enriching uranium to supplement that stock and give it a second path to the bomb.

North Korea's long-range rockets are not seen as capable of reaching the US mainland and it is not believed to have the technology to mount a nuclear warhead on a long-range missile.

The statement today appeared to dent any remaining hopes that Kim Jong-un, believed to be 30 years old, would pursue a different path from his father Kim Jong-il, who oversaw the country's military and nuclear programmes.

The older Kim died in December 2011.