US Defence Secretary Chuck Hagel said plans to bolster the country's missile defences were in response to "irresponsible and reckless provocations" by North Korea, which threatened a preventative nuclear strike against the US last week.
In addition to the 14 new anti-missile interceptors being added at Fort Greely in Alaska, the Pentagon has also left open the possibility of creating a site on the US East Coast where it could field more interceptors capable of striking down an incoming missile.
Hagel told a news conference, "By taking the steps I outlined today we will strengthen our homeland defence, maintain our commitments to our allies and partners, and make clear to the world that the United States stands firm against aggression".
Last week, North Korea issued a threat to stage a pre-emptive nuclear attack against the US as the United Nations readied new sanctions against Pyongyang in response to its nuclear test on February 12.
More top news
Andy Murray's hopes of a first French Open title remain intact after he pulled off another great escape against Radek Stepanek
Emergency services are at the scene of a "very serious" digger crash near Glasgow.
A woman who had just arrived back at Manchester Airport after a holiday abroad was surprised with a flash mob - and a wedding proposal.