WASHINGTON – Russian President Vladimir Putin bragged a year ago that his country had a new nuclear-powered missile with unlimited range. But the Kremlin will only produce a few of them because the weapon has yet to complete a successful test and is too expensive to develop, according to sources with direct knowledge of a U.S. intelligence report on the weapons program.

Russia's cruise missile Burevestnik, referred to as "Skyfall" in American intelligence reports, was tested once earlier this year. Prior to that, the weapon was tested four times between November and February in 2018, each resulting in a crash, according to sources who spoke to CNBC on the condition of anonymity.

The U.S. assessed 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 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 had boasted about.

The weapon, which has been in development since the early 2000s, is believed to use a gasoline-powered engine for takeoff before switching to a nuclear-powered one for flight, sources explained.

One U.S. intelligence report assesses that the Burevestnik will not be combat-ready for another decade, despite Putin's claim that the "invincible" weapon has a proven capability.