US Defence Secretary Chuck Hagel has announced plans to boost missile defences on the US West Coast to counter the threat from North Korea.
He said the US would add 14 interceptors, which can shoot down missiles in flight, to 30 already in place in California and Alaska by 2017.
Mr Hagel cited a “series of irresponsible and reckless provocations” recently by North Korea.
Tensions have risen after Pyongyang’s third nuclear test last month.
Only last week North Korea threatened the US with a pre-emptive nuclear strike.
‘Stay ahead of threat’
But despite the isolated country’s latest fiery rhetoric, analysts say the regime is years away from producing a missile with the capability to reach the continental United States.
“The US has missile defences to protect us from limited ICBM [Inter-Continental Ballistic Missile] attacks,” Mr Hagel told Friday’s press conference.
“But North Korea in particular has recently made advances in its capabilities and has engaged in a series of irresponsible and reckless provocations.”
The defence secretary said the US would also deploy a radar-tracking station in Japan.
Mr Hagel continued: “The reason we’re doing what we’re doing, and the reason we’re advancing our programme here for homeland security is not taking any chances, is to stay ahead of the threat and to ensure any contingencies.”
The Pentagon chief said the additional 14 interceptors would be deployed to Fort Greely, in the US state of Alaska, at a cost of about $1bn (Â£660m).
He also announced the Pentagon was beginning environmental impact studies for additional interceptor sites, allowing a shorter timeline for construction if the president decides to go ahead with installing further interceptors.
The Alaska and California sites were built during the presidency of George W Bush as protection from a possible strike by North Korea.
Technical difficulties with the interceptors slowed their installation.
When asked about the “poor performance” of interceptors during recent trials, Mr Hagel said further tests would be carried out this year.
“We have confidence in our system,” he said, “and we certainly will not go forward with the addition of the 14 interceptors until we’re sure we have the complete confidence we need.” (BBC)