The availability of ammunition will prove to be one of the most critical factors in Russia's ability to sustain its war in Ukraine, US officials told CNN. It has been difficult to assess Russia's levels of ammunition throughout the war, sources said, but its purchase of supplies from North Korea could suggest some Russian stocks are running low. US intelligence officials also believe that the Russians are running short on artillery shells and rockets, amid their scorched-earth tactics of unleashing artillery barrages on Ukrainian cities over the last six months.

For now, though, the ammunition deficit does not appear particularly widespread, officials said. But it may be particularly acute for select types of high-caliber multiple launch rocket systems, low caliber artillery systems and precision guided weapons, said Michael Kofman, the director of the Russia Studies Program at the Center for Naval Analyses. Kofman added that it will take time for Russia to meaningfully ramp up its production to meet the high volume of ammunition expenditure in the war, which the Russian defense industry clearly did not anticipate.

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In another signal that Moscow may be struggling to maintain the needed resources for its now six-month war, Russia has also been reallocating both manpower and equipment from far-flung military districts in both the east and the north, according to two sources familiar with US and western intelligence.

And Russia moved earlier this year to restrict public access to information about its budget, including import and export figures. According to one of these sources, that blackout is a key indicator that Russia has been forced to aggressively redirect funding and other resources towards the conflict -- a fact that it would want to hide from its public. The Kremlin continues to insist that it is not waging a war, but only a limited "special military operation."