WASHINGTON -- The Pentagon scored an important success Tuesday in a test of its oft-criticized missile defense program, destroying a mock warhead over the Pacific Ocean with an interceptor that is key to protecting U.S. territory from a North Korean attack.

This was the first test against a missile traveling at the trajectory and speed of an intercontinental ballistic missile -- a weapon which could threaten the U.S. mainland with a nuclear warhead.

North Korea continues developing shorter range missiles and its leader, Kim Jong Un, has announced he intends to launch an ICBM sometime this year, CBS News' national security correspondent David Martin reports.

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Vice Admiral Jim Syring, director of the Missile Defense Agency, called the test result "an incredible accomplishment" and a critical milestone for a program hampered by setbacks over the years.

"This system is vitally important to the defense of our homeland, and this test demonstrates that we have a capable, credible deterrent against a very real threat. I am incredibly proud of the warfighters who executed this test and who operate this system every day," Syring said in a statement announcing the test result.

Despite the success, the $244 million test didn't confirm that under wartime conditions the U.S. could intercept an intercontinental-range missile fired by North Korea. Pyongyang is understood to be moving closer to the capability of putting a nuclear warhead on such an ICBM and could develop decoys sophisticated enough to trick an interceptor into missing the real warhead.

Syring's agency sounded a note of caution.

An rocket designed to intercept an intercontinental ballistic missiles is launched from Vandenberg Air Force Base in Calif. on Tuesday, May 30, 2017. Matt Hartman/AP

"Initial indications are that the test met its primary objective, but program officials will continue to evaluate system performance based upon telemetry and other data obtained during the test," his statement said.

Before this latest test, the missile defense system had a track record of nine successful intercepts in 17 tries against other types of missiles. Despite that mediocre test record, Gen. Lori Robinson, head of Northern Command, which operates the missile defense system, has assured Congress it would work.

"I am extremely confident of our capability to defend the United States of America and be able to intercept an ICBM should it reach our homeland," Robinson said.

If it were the real thing, the U.S. would launch multiple interceptors against an incoming ICBM so if one missed, the second -- or third -- might hit it, Martin reports.

Ahead of Tuesday's test, Navy Capt. Jeff Davis, a Pentagon spokesman, said "This is part of a continuous learning curve."

The Pentagon is still incorporating engineering upgrades to its missile interceptor, which has yet to be fully tested in realistic conditions.

Missile defense is a work in progress, trying to stay ahead of the emerging threat of a North Korean ICBM. Since North Korea has not yet tested an ICBM, you would have to say that for now, the U.S. is ahead on points, Martin reports.

North Korea says its nuclear and missile programs are a defense against perceived U.S. military threats. Its accelerating missile development has complicated Pentagon calculations, most recently by incorporating solid-fuel technology into its rockets. The step would mean even less launch warning time for the U.S. Liquid fuel is less stable and rockets using it have to be fueled in the field, a process that takes longer and can be detected by satellites.

Underscoring its uninterrupted efforts, North Korea on Monday fired a short-range ballistic missile that landed in Japan's maritime economic zone.

In an interview with CBS News' "Face the Nation," U.S. Secretary of Defense James Mattis called North Korea a "direct threat to the United States."

"They have been very clear in their rhetoric we don't have to wait until they have an intercont- intercontinental ballistic missile with a nuclear weapon on it to say that now it's manifested completely," Mattis told "Face the Nation" host John Dickerson in his first official interview as defense secretary.

"But the bottom line is it would be a catastrophic war if this turns into a combat if we're not able to resolve this situation through diplomatic means," Mattis said.

In Tuesday's U.S. test, the Pentagon's Missile Defense Agency launched an interceptor rocket from an underground silo at Vandenberg Air Force Base in California. The target was an intercontinental-range missile fired from a test range on Kwajalein Atoll in the Pacific.

According to the plan, a 5-foot-long "kill vehicle" released from atop the interceptor zeroed in on the ICBM-like target's mock warhead outside Earth's atmosphere and obliterated it by sheer force of impact, the Pentagon said. The "kill vehicle" carries no explosives, either in testing or in actual combat.

The target was a custom-made missile meant to simulate an ICBM, meaning it flew faster than missiles used in previous intercept tests, according to Christopher Johnson, the Missile Defense Agency's spokesman. It was not a mock-up of an actual North Korean ICBM, and details of its exact capabilities weren't made public.

Officially known as the Ground-based Midcourse Defense system, the Pentagon likens the defensive tactic to hitting a bullet with a bullet. With congressional support, the Pentagon is increasing by the end of this year the number of deployed interceptors, based in California and Alaska, to 44 from the current total of 36.

While Tuesday's test wasn't designed with the expectation of an imminent North Korean missile threat, the military wants progress toward the stated goal of being able to shoot down a small number of ICBMs targeting the U.S.

Laura Grego, senior scientist at the Union of Concerned Scientists, which has criticized the missile defense program, called the interceptor an "advanced prototype," meaning it is not fully matured technologically even if it has been deployed and theoretically available for combat since 2004. A successful test Tuesday, she said, could demonstrate the Pentagon is on the right track with its latest technical fixes.

"Overall," she wrote in an analysis prior to the test, the military "is not even close to demonstrating that the system works in a real-world setting."

The interceptors are, in essence, the last line of U.S. defense against an attack by an intercontinental-range missile.

The Pentagon has other elements of missile defense that have shown to be more reliable, although they are designed to work against medium-range or shorter-range ballistic missiles. These include the Patriot missile, which numerous countries have purchased from the U.S., and the Terminal High-Altitude Area Defense, or THAAD, which the U.S. deployed this year to South Korea to defend against medium-range missiles from North Korea.