The United States announced on Friday that it was bolstering its military defences along the Pacific coast, including deploying more missiles in Alaska, in response to the growing threat from North Korea's nuclear programme.
A week after Pyongyang threatened to launch a pre-emptive nuclear strike on Washington, the Pentagon said it was increasing its fleet of interceptor missiles by nearly 50%.
Chuck Hagel, the newly appointed US defence secretary, said an additional 14 interceptors would be deployed to Alaska by the end of 2017, raising the total to 44 missiles stationed along the West coast.
"The United States has missile defence systems in place to protect us from limited intercontinental ballistic missile attacks, but North Korea in particular has recently made advances in its capabilities and is engaged in a series of irresponsible and reckless provocations," he said.
While North Korea is thought to be at least several years away from developing a capability to strike the US, Hagel said the Obama administration was determined to stay "ahead of the threat".
"Whatever their timelines are [the US must be sure] we're not reacting to those timelines and are ahead of any potential threat," he said.
Barry Pavel, a former senior director for defence policy at the White House's National Security Council, said the US sought to maintain a ratio of two interceptors for every potential incoming missile. By that formula, the expanded defence system would be able to intercept up to 22 missiles.
However, Pavel said that the Pentagon's announcement was pre-emptive and did not reflect the isolated communist dictatorship's current strength.
"They have a lot of technical efforts still ahead and I don't expect them to be capable any time soon, but perhaps within the next five years? It's possible," Pavel told The Daily Telegraph.
The ground-based interceptors have often proved inaccurate during testing but Hagel insisted "we have confidence in our system".
Last week, the North Korean foreign ministry threatened "pre-emptive nuclear strikes on the headquarters of the aggressors" after the UN voted to begin new sanctions in response to a nuclear test that the country conducted in February. The test, the country's third, drew international condemnation, including anger from China, North Korea's only major ally. In December, North Korea tested a long-range rocket in defiance of a UN ban.
Hagel also said the US would deploy a new radar system in Japan that would give "improved early warning and tracking of any missile launched from North Korea at the United States or Japan".
He said that sending additional resources to the Pacific would not impact on plans to cover all of Europe under a Nato missile shield by 2018.
"Let me emphasise the strong and continued commitment of the United States to Nato missile defence. That commitment remains ironclad," Hagel said.
The US is undertaking studies as it decides whether to deploy an additional interceptor site, possibly on the Atlantic coast, to counter any future threat from Iran.
"While the administration has not made any decision on whether to proceed with an additional site, conducting environmental impact studies will shorten the timeline for construction should that decision be made," Hagel said.
The new Pacific interceptors are expected to cost about $200 million and are being deployed at a time of significant cutbacks at the Pentagon.
Hagel said the new missile deployments were designed to "counter future missile threats from Iran and North Korea, while maximising scarce taxpayer resources".