The finding is likely to provide fodder for Republican politicians who oppose additional aid for Kyiv, but the Pentagon said there is no evidence that military assistance provided to Kyiv has been illicitly diverted.

"As of June 2, 2023, serial number inventories for more than $1.005 billion (59 percent) of the total $1.699 billion of EEUM-designated defense articles were delinquent," the inspector general's office said in a statement, referring to enhanced end-use monitoring.

The shortfall can be explained by factors including the "limited number of US personnel at logistics hubs in a partner nation and in Ukraine," and restrictions on the movement of monitoring personnel in the country, the statement said.

When a serial number inventory is conducted, officials view the item and write down or scan its barcode, then update that information in a database, according to an official from the inspector general's office.

The full inspector general's report says that "high rates of delinquency may correlate with an inability to maintain complete accountability of the EEUM?designated defense articles, which, in turn, may increase the risk of theft or diversion."

Equipment that has been provided to Kyiv and which requires enhanced end-use monitoring includes Javelin anti-tank weapons, Stinger anti-aircraft missiles and Switchblade one-way attack drones.

Pentagon spokesman Major General Pat Ryder told journalists Thursday that there have been no signs of misuse of the aid Washington has provided.

"There remains no credible evidence of illicit diversion of US-provided advanced conventional weapons from Ukraine," he said.

Washington has spearheaded the push for international support for Ukraine since Russia invaded in February 2022, forging a coalition to back the country and coordinating tens of billions of dollars in aid that has helped Kyiv's forces push Moscow's troops back.

While US authorities have authorization to withdraw more military equipment for Ukraine from American stockpiles, "we don't have the funds available to us to replenish those stocks," Ryder said.

Republicans have refused to authorize new budget outlays for Ukraine unless Democrats first agree to sweeping, tough new measures to curb illegal immigration and tighten the asylum process.