More than $1 billion worth of shoulder-fired missiles, kamikaze drones and night-vision goggles that the United States has sent to Ukraine have not been properly tracked by American officials, a new Pentagon report concluded, raising concerns that they could be stolen or smuggled at a time when Congress is debating whether to send more military aid to Kyiv.

The report by the Defense Department’s inspector general, released on Thursday, offers no evidence that any of the weapons have been misused after being shipped to a U.S. military logistics hub in Poland or sent onward to Ukraine’s front lines.

But it found that American defense officials and diplomats in Washington and Europe had failed to quickly or fully account for many of the nearly 40,000 weapons that by law should have been closely monitored because their battlefield impact, sensitive technology and relatively small size makes them attractive bounty for arms smugglers.

“These are identified as the items — that because of their sensitivity, their vulnerability to diversion or misuse or the consequences of that — it’s particularly important to have this additional tracking and accountability in place,” Robert P. Storch, the Pentagon’s inspector general, who is also the lead watchdog for American aid sent to help Ukraine’s war effort, said in an interview on Thursday.

The report was sent to Congress on Wednesday and a copy of it was provided to The New York Times. The Pentagon’s inspector general released a redacted version of it on Thursday. It did not investigate whether any weapons had been diverted for illicit use, which “was beyond the scope of our evaluation to determine,” it noted.

The number of weapons reviewed in the report represents only a small fraction of about $50 billion in military equipment that the United States has sent Ukraine since 2014, when Russia seized Crimea and parts of the eastern Donbas region. Most of the weapons that have been delivered so far — including tanks, air-defense systems, artillery launchers and ammunition — were pledged after Russia’s full-scale invasion in February 2022.

Yet the investigation offers a first glimpse of efforts to account for the most sensitive tools of American military might that have been rushed to Ukraine in the last two years. In that time, as concerns grew that the flood of weapons would inevitably lead to arms trafficking, lawmakers have demanded strict oversight of the shipments.

The findings released on Thursday will almost certainly fuel skepticism in Congress over providing more military aid to Ukraine; already, House Republicans are blocking a national security spending plan that would provide an additional $61 billion for the war effort as frontline troops begin to run out of weapons. Combined with Ukraine’s long history of corruption and arms smuggling, the demand for closer accounting is certain to rise.

The report did not detail exactly how many of the 39,139 high-risk pieces of matériel that were given to Ukraine were considered “delinquent,” but it put the potential loss at about $1 billion of the total $1.69 billion worth of the weapons that had been sent.

As of last June, the latest data available, the United States had given Ukraine nearly 10,000 Javelin anti-tank missiles, 2,500 Stinger surface-to-air missiles and about 750 kamikaze Switchblade drones, 430 medium-range air-to-air missiles and 23,000 night-vision goggles. It also provided launcher parts for the Javelins and Stingers that were to be kept in stockpiles even after the missiles were fired.

As much as 60 percent of the arms and equipment that were provided as of June were “delinquent,” either because they were delayed in being inventoried in a database designed to track them, or because they were never added after they left American or allied military stockpiles.

Pentagon and State Department officials and other experts have long held that it is nearly impossible to account for each of the thousands of weapons that have been sent to Ukraine. The chaos of combat, the risk of traveling to battlefields and the lack of staff in Washington, Kyiv and at a logistics hub in Poland have all hindered close oversight, although officials maintain there has been no sign of wide-scale smuggling or mishandling of the arms since the war began.

The required accounting procedures “are not practical in a dynamic and hostile wartime environment,” Alexandra N. Baker, the acting under secretary of defense for policy, wrote in a Nov. 15 response to an earlier draft of the report.

Ms. Baker also said there were not enough Defense Department employees at the U.S. Embassy in Kyiv to easily track all of the most sensitive weapons and equipment, which she said currently totaled more than 50,000 items in Ukraine “and growing.” (The reporting requirements for weapons were suspended in the days before the invasion on Feb. 24, 2022, when the U.S. Embassy in Kyiv was evacuated, and didn’t resume until the embassy reopened in May.)

In the earliest days of the war, and as a column of Russian tanks headed for Kyiv, Ukrainian forces frantically deployed Javelin missiles and other weapons to defend themselves — most likely without stopping to inventory them, officials have conceded.

But in at least some instances, according to Thursday’s report, Ukrainian military officials took better care to track the arms and equipment than their American counterparts.

In one example, of a sample of 303 pieces of equipment sent to Ukraine between February 2022 and March 2023, the report found that American officials had accounted for 47 of them while passing through logistics centers in Poland, and inventoried 15 that had arrived in Ukraine.

By comparison, the report found, Ukrainian officials were able to account for 73 pieces of equipment — meaning they were more assiduous about updating their inventories.

Mr. Storch, the inspector general, said U.S. officials had impressed “the importance of ensuring appropriate accountability for the equipment” upon Ukrainian forces. The process became easier in late 2022, he said, when Ukrainian troops were given hand-held bar code scanners to instantly transmit the serial numbers of advanced weapons into American databases.

The new process was part of the decision by the Biden administration to give Ukraine more authority to self-report how it is securing arms. But only 10 scanners have been given to Ukrainian forces, the report said, and none of them are on the front lines.

Currently, only two inspectors from Mr. Storch’s office are working at the U.S. Embassy in Kyiv, although officials are considering sending at least two more to help with the oversight.

But battlefield realities have made the task increasingly difficult. Ukrainian troops often swap their stockpiles of Javelin and Stinger missiles with other units for specific equipment needed to fight the Russians on their portion of the front, opening the potential for possible diversion to illicit third parties. “It’s tough to do in a wartime setting,” Mr. Storch said. “There’s still room for improvement, even on the Ukrainian side.”

Thomas Gibbons-Neff contributed reporting.