US Defence Secretary Chuck Hagel has announced plans to bolster US missile defenses in response to a growing nuclear threat from North Korea.

He said the US will add 14 interceptors to a missile defence site in Alaska by 2017.

The US will also deploy a radar tracking system in Japan.

The decision to add 14 new anti-missile interceptors at Fort Greely in Alaska amounts to a reversal of an Obama administration decision in 2010.

The decision was to stop the expansion of the missile field there at 30 interceptors.

The Bush administration had planned to deploy 44 total interceptors.

Mr Hagel said the decision to deploy all 44 interceptors came as a result of the growing threats from Iran and particularly North Korea.

North Korea tested a third nuclear device last month and launched a rocket that put a satellite in orbit in December.

He did not say when the deployment of additional interceptors would start.

Mr Hagel also said the US would move forward with a plan announced by his predecessor last year to deploy a second missile defense radar in Japan.

The announcement came a week after North Korea threatened the US with a pre-emptive nuclear strike.

Experts say North Korea is years away from being able to hit the continental US with a nuclear weapon despite a decades-long push toward a nuclear capability.