As the first winter of Russia’s war with Ukraine approaches, the U.S. is running out of the latest weapons systems and ammunition to transfer to Kyiv, three U.S. officials told CNN.

Pressure on arms inventories — and the ability of the U.S. industrial base to keep up with demand — is one of the key challenges facing the Biden administration as the U.S. continues to send billions of dollars worth of weapons to Ukraine to support its fight against Russia. One official said stocks of certain systems are “low” after nearly nine months of military aid to Kiev during a high-intensity war, as the US is prepared to send a “limited amount” of surplus supplies.

So, sources say, 155 mm artillery ammunition and Stinger anti-aircraft missiles are causing concern.

Some sources also expressed concern about US production of additional weapons systems, including HARM anti-radiation missiles, GMLRS surface-to-surface missiles, and Javelin man-portable anti-tank missiles, although the US has decided to increase production of these systems for other systems as well.

For the first time in two decades, the US is not directly involved in the conflict after withdrawing from Afghanistan and moving into an advisory role in Iraq. Without the need to produce weapons and munitions for war, the US did not produce the quantities of materials needed to sustain a prolonged, high-intensity conflict.

Defense officials say the crisis does not affect U.S. readiness to intervene in the conflict because the weapons sent to Ukraine do not come from what the U.S. keeps for its own troops.

Several officials emphasized that the U.S. would never compromise its own readiness, and each delivery would be measured by its impact on U.S. strategic reserves and military plans. According to officials, Defense Secretary Lloyd Austin and the chairman of the Joint Chiefs of Staff, General Mark Milley, are closely monitoring the stockpile levels in the United States.

One reason for concern about low inventories is that the U.S. industrial base is struggling to keep up with demand, the sources said.

In addition, European allies cannot sufficiently supplement Ukrainian military requirements due to the need to maintain the supply of their own forces.

“It’s getting harder and harder,” House Intelligence Committee member Mike Quigley told CNN. At a time when global supply chains are melting down, it will be very difficult for the West to meet demand at such a high level.”

Colin Kahl, the Pentagon’s undersecretary for policy, recently told reporters that there is “no question” that arms shipments to Ukraine are putting pressure on the U.S. stockpile and industrial base, as well as on its allies.