A nuclear-powered missile built by Russia that is believed to have "unlimited range" could be ready for war within the next six years, a U.S. intelligence assessment claims.

Sources with knowledge of the assessment told CNBC Wednesday that the timeline is slightly accelerated to what the Kremlin had previously reported. It also comes despite a slew of unsuccessful tests of the weapon.

Russian President Vladimir Putin touted a new nuclear-powered missile last year that he said would render NATO’s American-led missile defense system “useless.” He claimed at the time the missile had unlimited range, flew at a high speed and was capable of maneuverability that would allow it to pierce any missile defense.

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Since then, the missile – which Russia calls the 9M730 Buresvestnik--has been unsuccessfully tested several times, including earlier this year. Each test resulted in a crash.

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U.S. officials determined the longest test flight lasted just more than two minutes, with the missile flying 22 miles before crashing, CNBC reported.

While Putin boasted of the ability to strike long-range targets, two U.S. officials briefed on Moscow's military capabilities told Fox News at the time the weapon was not operational yet and is still in its “nascent" stage.

“The nuclear-powered cruise missile Putin bragged about has actually crashed a few times,” during recent testing in the Arctic, one official said. “Think about the environmental impact of that."

Both agreed to speak only on the condition of anonymity because they were not authorized to speak with the media.

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The news on an accelerated timeline for the weapon’s readiness comes after a mysterious explosion in northern Russia last month that left at least five people dead and unleashed a radiation spike. U.S. officials told the New York Times after the blast that it involved a prototype of the Buresvestnik, also known as Skyfall.

Fox News' Travis Fedschun contributed to this report.