As winter approaches, the U.S. is running out of some high-tech weapons systems and ammunition to transfer to Kyiv, CNN reports, citing three U.S. officials.

The strain on weapons stocks and the ability of the U.S. industrial base to meet demand is one of the key issues facing the Biden administration as the U.S. continues to send billions of dollars worth of weapons to Ukraine. One official said the stockpile of some systems is "depleting" because there is a "finite amount" of inventory the U.S. can send.

Sources said 155-mm artillery ammunition and Stinger portable anti-aircraft missiles are among the weapons systems of particular concern. Some sources also expressed concern about U.S. production of additional weapons systems, including HARM anti-radioactive missiles, GMLRS ground-to-ground missiles, and Javelin portable anti-tank missiles, although the U.S. has taken steps to ramp up production of these and other systems.

Without the need to produce weapons and ammunition for war, the U.S. has not produced the amount of materiel needed to sustain a sustained, high-intensity conflict.

Defense Department officials say the crisis does not affect U.S. combat readiness because the weapons sent to Ukraine do not come from U.S. contingency stocks.

But the seriousness of the problem is a source of debate within the Defense Department, officials say. While the U.S. cannot indefinitely supply Ukraine with high-quality ammunition, assessing whether the U.S. stockpile is "depleted" is subjective, one senior Defense Department official said, because it depends on how much risk the Pentagon is willing to take.

Several officials stressed that the U.S. would never put its own combat readiness at risk, and each shipment is evaluated in terms of its impact on U.S. strategic reserves and military plans. According to the officials, Defense Secretary Lloyd Austin and Chairman of the Joint Chiefs of Staff Gen.