The Pentagon has a funding shortfall of about $10 billion for U.S. military weapons needed to replace those already sent to Ukraine, a shortfall that requires additional money from Congress to fix, top Defense Department officials said Monday.

“We are probably looking at about $10 billion to replace everything, everything that we’ve given in terms of supplies to Ukraine,” one official told VOA.

“We don’t foresee a likely alternative outside of the supplemental funding [bill] or having that money added into an appropriations bill in order to achieve the replenishment that we need,” Deputy Secretary of Defense Kathleen Hicks told reporters.

Pentagon officials expected to get the funding to replenish those stocks in a supplemental request from the Biden administration, which included billions of additional dollars in aid for Ukraine, Israel and Taiwan. However, Congress has yet to pass a supplemental aid bill because of arguments on spending and U.S. border security.

The shortfall is tied to the way the Pentagon has accounted for the aid sent to Ukraine. Last June, the Pentagon said it overestimated the value of weapons sent to Ukraine by about $6.2 billion over the past two years.

When calculating its aid package estimates, the Department of Defense was counting the cost incurred to replace the weapons given to Ukraine, while it said it should have been totaling the cost of the systems actually sent, officials told VOA at the time.

The error provided the Pentagon the legal cover needed to send more aid to Ukraine, but the problem remained that more funds would be needed to replenish U.S. military stockpiles with newer, costlier weapons.

Failing to replenish U.S. stockpiles would negatively affect the military’s readiness, another defense official told VOA.

The department still has about $4 billion in authority to send aid to Ukraine, but Pentagon officials have told reporters that sending additional aid without the ability to replenish U.S. weapons stockpiles would be a risk the Defense Department is not willing to take at this time.

But Retired Rear Admiral Mark Montgomery, a defense analyst with the Foundation for Defense of Democracies, told VOA there are key weapons the U.S. could send Kyiv without compromising military readiness, “because the Army no longer needs them.”

One weapon would be 155-millimeter cluster munitions, which Montgomery says the U.S. doesn’t use in combat planning.

“That would give 155-mm ammunition to the Ukrainians very quickly,” he said.

Another weapon that could be immediately sent without incurring a cost on U.S. military readiness would be M113 armored vehicles.

“We have thousands of them that we’re getting rid of. We could transfer these to Ukraine,” he told VOA.