The US military scored an important success in a test of its oft-criticised missile-defence programme, destroying a mock warhead over the Pacific Ocean with an interceptor that is key to protecting US territory from a North Korean attack.

Vice Admiral Jim Syring, director of the Pentagon agency in charge of developing the missile-defence system, called the test result "an incredible accomplishment" and a critical milestone for a programme hampered by setbacks over the years.

"This system is vitally important to the defence of our homeland, and this test demonstrates that we have a capable, credible deterrent against a very real threat," Syring said in a statement announcing the test result.

Despite the success, the $244m test didn't confirm that under wartime conditions the US could intercept an intercontinental-range missile fired by North Korea.

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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.

The most recent intercept test, in June 2014, was successful, but the longer track record is spotty. Since the system was declared ready for potential combat use in 2004, only four of nine intercept attempts have been successful.

Failure on Tuesday could have deepened concern about a programme that, according to one estimate, has so far cost more than $40bn.

John Tierney, executive director of the Center for Arms Control and Non-Proliferation, told Al Jazeera the missile programme - despite its cost - has a success record of less than 50 percent.

"Unfortunately, we need to be aware of a false sense of security here. This programme is nowhere near ready to be relied upon against North Korea or anybody else. This is a baby step," Tierney said.

Pentagon spokesman Navy Captain Jeff Davis had said the test was not timed specifically in response to tensions with Pyongyang but "in a broad sense, North Korea is one of the reasons why we have this capability".

North Korea says its nuclear and missile programmes are a defence against perceived US 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 United States. 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 Tuesday's US 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, two-metre-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.

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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.

Laura Grego, senior scientist at the Union of Concerned Scientists, which has criticised the missile defence programme, called the interceptor an "advanced prototype", meaning it is not fully matured technologically.

"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".