The Pentagon also left open the possibility of creating a site on the US East Coast where the Pentagon could field more interceptors capable of striking down an incoming missile. The 14 additional interceptor deployments would cost nearly $1 billion and must be approved by Congress.
"By taking the steps I outlined, we will strengthen our homeland defence, maintain our commitments to our allies and partners, and make clear to the world that we stand firm against aggression," Hagel said.
North Korea issued its threat last week to stage a preemptive nuclear attack against the United States as the UN readied new sanctions against Pyongyang in response to its February 12 nuclear test, the country's third.
Experts say North Korea is years away from being able to hit the continental US with a nuclear weapon, despite having worked for decades.
But Hagel said the moves announced by the Pentagon were justified to stay ahead of the threat, underscored by the nuclear test and a December rocket launch that analysts believe was aimed at developing technology for an intercontinental ballistic missile (ICBM).