Since the start of the war, the US has transferred 806,000 shells of 155mm howitzers and 108,000 shells for 105mm guns, according to the Defense Department figures. That's nearly one million shells in six months, and the figure does not factor in missiles and other precision-guided rockets.

As of June, Ukrainian forces fired about 6,000 shells per day at Russian forces. Artillery in warfare is very important, but there's no way Ukraine can manufacture such shells at scale, so the US is quickly burning through its ammunition stockpile. 

US defense officials told WSJ that stockpiles of 155mm shells are "uncomfortably low" after months and months of arming Ukraine. 

The Pentagon has been slow to replenish its arsenal and may jeopardize readiness for a broader conflict with top superpowers, such as Russia and China. 

That's why Secretary of Defense for Acquisition and Sustainment William LaPlante revealed Friday that the US plans to increase the production capacity of 155mm ammunition from 14,000 to 36,000 shells per month over the next three years. 

"Right now, the 155 [mm] munitions are [produced] at about 14,400 a month, and we have plans working with the contractor to get that in increments ultimately up to 36,000 a month by about three years. It's going to be in steps," LaPlante said during a press conference.