Despite a slew of unsuccessful tests, Russia's nuclear-powered missile with so-called unlimited range will be ready for war within the next six years, a slightly accelerated timeline than previously reported, according to a U.S. intelligence assessment.

The revelation of the new, more ambitious timeline for the missile comes even though the Kremlin has yet to secure a successful test over multiple attempts, according to sources with knowledge of a U.S. intelligence report, CNBC said.

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It also comes on the heels of a mysterious explosion off Russia's northern coast that killed five scientists and sparked fears Moscow had tested the missile in question, called Burevestnik. A U.S. intelligence assessment found that the August 8 explosion occurred during a recovery mission to salvage a lost Burevestnik from the ocean floor.

Last March, Russian President Vladimir Putin unveiled several hypersonic weapons, as well as Burevestnik. Putin said it was nuclear powered and had unlimited range.

Burevestnik, also known as Skyfall, has been tested once earlier this year and prior to that, the weapon was tested four times between November 2017 and February 2018, each resulting in a crash.

The U.S. determined that the longest test flight lasted just more than two minutes, with the missile flying 22 miles before losing control and crashing. The shortest test lasted four seconds and flew for five miles. The tests apparently showed that the nuclear-powered heart of the cruise missile failed to initiate and, therefore, the weapon was unable to achieve the indefinite flight Putin bragged about.