Concerned by North Korea's long-range missile advances, Defense Secretary Chuck Hagel announced today that the United States is bolstering its missile defense system by adding 14 more ground-based missile interceptors.

The additional missiles will be in place by 2017 and will bring the number of long-range missile interceptors from 30 to 44, an almost-50-percent increase. Currently, 26 interceptors are based at Fort Greely, Alaska, and four more at Vandenberg Air Force Base, Calif.

Hagel cited recent North Korean actions as a motivator behind today's announcement and said the United States wants to "stay ahead" of any missile threats posed by North Korea and not have to react to North Korean developments.

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Adding the 14 missiles "gives our country the security it needs and the people need to be reassured that that security is there," he said.

North Korea, he said, "recently made advances in its capabilities and is engaged in a series of irresponsible and reckless provocations."

He noted North Korea's third nuclear test in February, December's launch of a long-range missile that placed a satellite in orbit, and last year's public display of a road-mobile intercontinental ballistic missile known as the KN-08.

He expressed confidence in the ground-based missile interceptors that have been operational for more than eight years but have had some problems during tests.

"We are sure that we have the complete confidence that we will need," said Hagel. "But the American people should be assured that our interceptors are effective."

Hagel told reporters that the rationale for "advancing our program here for homeland security is to not take any chances, is to stay ahead of the threat and to assure any contingency. And that's why we've made the decisions that we have."

He added that the United States did not want to be in a position to react to North Korean missile development timelines but "that we're ahead of any timelines of any potential threat."

Deploying the additional missiles will cost less than $1 billion and will likely begin after another missile test that is scheduled for the fall.

Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff, told reporters at a Pentagon briefing that the U.S. has been watching North Korea's missile developments "very, very closely " and that the U.S. missile defense program could always be tweaked, if needed.

"So we have continually built this hedge, a set of tools from which we can select if the threat either goes faster or slower than we thought," he said. "And so the Korean threat went just a little bit faster than we might have expected. [We] very simply pulled the tools off the shelf, and those four tools are what we're announcing today."

In addition to 14 more missile interceptors, Hagel also included a previously announced move to send an additional TPY-2 early warning radar to Japan. He added that there would be a two-year delay in the roll-out of the SM3-IIB missile as part of the European missile defense shield designed to counter a potential threat to Europe posed by Iranian long-range missiles.

Hagel also announced that the Pentagon would be begin environmental impact studies to add more ground-based missile sites in the United States. The administration has not made a decision about an additional site, but Congress inserted language in this year's defense authorization bill that required the placement of such sites along the eastern seaboard of the United States.

The KN-08 mobile missile has been of particular concern to U.S. officials since six of the missiles were first displayed on mobile transports during a parade in Pyongyang last April. The mobility offered by such a launcher system concerns U.S. intelligence and military officials because it makes tracking the missiles more difficult, especially if they are being prepared for a potential launch.

Though U.S. officials were uncertain as to whether the displayed North Korean missile may have been a fake, Winnefeld said it "probably does have the range to reach the United States." He could not provide additional details because, he said, the intelligence assessment of where the North Koreans are in their development of the missile's launch capabilities remains classified.

On Monday, National Security Adviser Tom Donilon told the Asia Society that the U.S. will not "stand by while [North Korea] seeks to develop a nuclear-armed missile that can target the United States." He said the U.S. remained committed to ensuring peace on the Korean peninsula and, "this means deterring North Korean aggression and protecting our allies."

Donilon said the international community "has made clear that there will be consequences for North Korea's flagrant violation of its international obligations."

Last week, in fact, the U.N. Security Council unanimously approved new sanctions on North Korea after the nuclear test.

Winnefeld cited Donilon's comments about deterrence in explaining today's announcement.

"I think the national security adviser made it very clear in his speech on Monday that we not only intend to put the mechanics in place to deny any potential North Korean objectives to launch a missile to the United States, but also to impose costs upon them if they do," said Winnefeld. "And we believe that this young lad [North Korean leader Kim Jong Un] ought to be deterred by that. And if he's not, we'll be ready."