The United States is to bolster its defenses against a possible North Korean missile strike by siting 14 more interceptors in Alaska, Defense Secretary Chuck Hagel announced Friday.

The new batteries will boost by almost half the 30 interceptors already deployed along the California and Alaska coastline and should be in place by 2017, as Pyongyang races to develop a nuclear-armed ballistic threat.

North Korea has tested nuclear devices, and Hagel said the US defensive upgrade was designed to "stay ahead of the threat" as Kim Jong-un's pariah state attempts to marry its bombs to its missile technology.

"The United States has missile defense systems in place to protect us from limited ICBM attacks," Hagel said.

"But North Korea in particular has recently made advances in its capabilities and is engaged in a series of irresponsible and reckless provocations."

Hagel cited the North's third nuclear test last month, the display of a suspected mobile intercontinental ballistic missile (ICBM) in April and the launch of a satellite into orbit as causes for concern.

Apart from mobilizing the additional interceptors at Fort Greely, Alaska, Hagel also confirmed an announcement from last year that the United States plans to deploy a second advanced radar to Japan.

Hagel also said the Pentagon was conducting environmental impact studies to clear the way for a possible additional American site for a ground-based missile interceptor, but not location has yet been chosen.

America's missile defense program has been plagued by technical problems, with the interceptors often failing in test launches, but Hagel said: "The American people should be assured that our interceptors are effective."

North Korea has missiles that can strike South Korea and Japan but has yet to demonstrate it has the capability to fire long-range missiles that could reach the United States.

Officials worry, however, that the North has made progress on the ICBM front, having put a satellite into orbit, while Pyongyang's nuclear weapons program heightens concerns about the threat posed by the hermit nation.

North Korea has issued a wave of drastic threats in recent days and announced it has abandoned the 60-year-old armistice that ended the Korean War.

The North's military fired short-range missiles into the East Sea (Sea of Japan) on Friday, Yonhap reported, a day after Kim presided over a live-fire artillery drill near the disputed Yellow Sea border with South Korea.