As the first full winter of Russia’s war with Ukraine approaches, the US is running low on high-end weapons systems and ammunition available for transfer to Kyiv, according to three US officials with direct knowledge.

One of the key challenges facing the Biden administration is the strain on weapons stockpiles, as well as the ability of the US industrial base to keep up with demand, as the US continues to send billions of dollars in weapons to Ukraine to support its fight against Russia. After nearly nine months of sending supplies to Kyiv during the high-intensity war, one of the officials said the stockpiles of certain systems are “dwindling,” as the US has a “finite amount” of excess stocks available to send.

According to the sources, 155mm artillery ammunition and Stinger anti-aircraft shoulder-fired missiles are among the weapons systems where there is particular concern about US stockpiles meeting Ukrainian demands.

Some sources also expressed concern about the US production of additional weapons systems, such as HARMs anti-radiation missiles, GMLRS surface-to-surface missiles, and portable Javelin anti-tank missiles, despite the fact that the US has moved to increase production of those and other systems.

After withdrawing from Afghanistan and transitioning to an advisory role in Iraq, the United States is no longer directly involved in a conflict for the first time in two decades. Without the need to manufacture weapons and ammunition for a war, the United States has not produced the quantities of materiel required to sustain a long-term, high-intensity conflict.

Defense officials say the crisis isn’t affecting US readiness because the weapons sent to Ukraine aren’t from what the US keeps on hand for its own contingencies.

Officials say the severity of the problem is being debated within the Defense Department. While the US will not be able to provide high-end munitions to Ukraine indefinitely, one senior defense official stated that determining whether the US is “running low” on stockpiles is subjective, as it depends on how much risk the Pentagon is willing to take.

Multiple officials emphasized that the US would never jeopardize its own readiness, and that each shipment is evaluated in terms of its impact on US strategic reserves and war plans. Officials said that Defense Secretary Lloyd Austin and Chairman of the Joint Chiefs of Staff Gen. Mark Milley closely monitor US stockpile levels.

According to the sources, one reason for the concern about low stockpiles is that the US industrial base is having difficulty keeping up with demand quickly enough. Furthermore, European allies are unable to adequately backfill Ukrainian military requests due to the need to maintain their own forces’ supplies.

Austin touted the commitments of a half-dozen countries providing additional weapons to Ukraine at a press conference Wednesday following a meeting of the Ukraine Defense Contact Group, including Greece pledging more 155mm ammunition.

The extent to which weapons stockpiles are depleted varies by system, as the US defense industrial base is better equipped to ramp up production of some weapons, while others are more difficult – or the production line has been shut down entirely and cannot easily be restarted.

In a September fact sheet, the Pentagon stated that it had committed more than 806,000 155mm artillery rounds to Ukraine. According to Cancian, the ammunition for the 155mm howitzers was “probably close to the limit that the United States is willing to give without jeopardizing its own warfighting capabilities.” At the same time, he wrote that a dozen other countries could supply the same ammunition, and Ukraine was unlikely to be constrained in what it needed thanks to the global market.

In a recent roundtable, Colin Kahl, the Pentagon’s undersecretary for policy, told reporters that “there’s no doubt” the weapons pipeline to Ukraine has put pressure on the US stockpiles and industrial base, as well as those of its allies.

Kahl added that the US support for Ukraine has not put the US military “in a dangerous position as it relates to another major contingency somewhere in the world,” but it has revealed that more work needs to be done to ensure the US defense industrial base is nimbler and more responsive.