WASHINGTON (AP) — Shortfalls in required monitoring by American officials mean the U.S. cannot track more than $1 billion in weapons and military equipment provided to Ukraine to fight invading Russian forces, according to a Pentagon audit released Thursday.

The findings mean that 59% of $1.7 billion in defense gear that the U.S. has provided Ukraine and was directed to guard against misuse or theft remained "delinquent," the report by the Defense Department's office of the inspector-general, the watchdog body for the Pentagon, said.

While Biden administration officials stressed Thursday that there was no evidence the weapons had been stolen, the audit undermines two years of lavish assurances from the administration that rigorous monitoring would keep U.S. military aid given to Ukraine from being misused. That's despite the country's longstanding reputation for corruption.

"There remains no credible evidence of illicit diversion of U.S.-provided advanced conventional weapons from Ukraine," Brig. Gen. Patrick Ryder, a Pentagon spokesman, told reporters. Citing what he said was Russian disinformation to the contrary, Ryder added, "The fact is, we observed the Ukrainians employing these capabilities on the battlefield. We're seeing them use them effectively."

President Joe Biden is already struggling to win congressional approval for more U.S. military and financial aid to Ukrainian government forces, which are struggling to drive out Russian forces that pushed deeper into the country in February 2022. The audit findings are likely to make Biden's task even harder.

House Republican opposition for months has stalled Biden's request to Congress for $105 billion more for Ukraine, Israel and other national security objectives. National Security Council spokesman John Kirby said Thursday that there was no funding left for additional military aid packages to Ukraine.

The U.S. has provided tens of billions of dollars in military aid to Ukraine, including big systems such as air defense. The end-use monitoring was required for gear that had sensitive technology and was smaller, making it more vulnerable to arms trafficking.

The Pentagon inspector general's report said that the Defense Department had failed to maintain an accurate serial-number inventory of those defense articles for Ukraine as required.

Reasons for the shortfall in monitoring included limited staffing; the fact that procedures for carrying out end-use monitoring in a war zone weren't put in place until December 2022; restrictions on movement for monitors within Ukraine; and a lack of internal controls on inventory, the report said.

While the U.S. had improved monitoring since the first year of the war, "significant personnel limitations and accountability challenges remain," auditors said; full accounting of the gear was impossible as long as those shortfalls remained, they said.

Kirby said administration officials "has for many months now been interested in improving accountability over the end use of material that is provided to Ukraine."

The audit didn't attempt to determine whether any of the assistance had been diverted. It noted the Defense Department inspector-general's office now had people stationed in Ukraine, and that its criminal investigators were still looking into allegations of criminal misuse of the security assistance.

Defense Department officials told auditors they expected to have systems for improved oversight in place this year and next.

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Pentagon reporters Lolita C. Baldor and Tara Copp contributed.