(NewsNation) — As the U.S. sends more and more weapons into Ukraine, there are new concerns some are getting into the wrong hands.

According to multiple reports, Russia is capturing some U.S. and NATO-provided weapons left on the battlefield and sending them to Iran where they could reverse engineer the systems.

On the record, Department of Defense officials aren’t mentioning any “significant evidence of diversion,” but say they wouldn’t be surprised if was happening.

Officials have witnessed instances, according to reports, of Russian forces seizing weapons like Javelin anti-tank and Stinger anti-aircraft systems.

The Pentagon told NewsNation they are aware of the reporting, but were unable to comment further, instead pointing to a defense official’s recent testimony on Capitol Hill.

“What we’re not seeing is any evidence of significant diversion,” said Colin Kahl, Under Secretary of Defense for Policy, testifying during a House Armed Services Committee hearing on oversight of U.S. military support to Ukraine.

“I think our assessment, if some of these systems have been diverted, it’s by Russians who have captured things on the battlefield,” Kahl said. “Which always happens, but that there’s no evidence the Ukrainians are diverting it to the black market.”

A State Department spokesperson told NewsNation “battlefield losses occur in every conflict, and we cannot speak for what Russia does with arms it captures.”

Ukrainians have handheld scanners where they keep track of weapon inventories. The data gets transmitted directly back to the U.S. so they can prevent any potential diversion.

But this isn’t the first time concerns have been raised over weapons falling into adversaries’ hands.

NewsNation reported on the growing fear over U.S.-provided weapons ending up in the wrong hands back in August.

One expert said it was inevitable.

“The scale is what’s hard to know right now,” said Jeff Abramson, senior fellow at the Arms Control Association. “And that’s why there’s this big push for oversight and monitoring now so that we can get a handle on that.”

David Tafuri, former Obama campaign foreign policy advisor and former State Department official, says Russia and Iran have drawn closer and it would be “problematic” if Iran was able to reverse engineer some of the U.S. weapons systems.

“It would be a problem for us security-wise, if Iran were able to reverse engineer a Javelin. It’s been very successful and effective on the ground in Ukraine, and we don’t want that to happen.”

Tafuri said Iran is very focused on developing its conventional weapons.

“Who knows what capability Iran already has?” Tafuri said. “It has been effective in developing drones that obviously have been useful to Russia unfortunately in Ukraine. It is also using those drones in the Middle East and notably, including in places like Iraq, and even Saudi Arabia.”

Calls are growing on Capitol Hill for more oversight over the billions of dollars in weapons and equipment flowing into Ukraine.

Pentagon Inspector General Robert Storch was pressed early by Rep. Andrew Clyde, R-Ga., on instances of U.S.-provided weapons being diverted.

“So you can confidently say that to your knowledge every sensitive weapon is currently under control of those who should have them?” Clyde asked.

Storch responded, “We’re laser-focused on this issue and engaged in robust oversight to make sure that’s the case.”

The State Department also warned Russia could be pushing out false reports of alleged illicit arms diversion as an attempt to weaken Western support for Ukraine.

Some Republicans in Washington are going as far as calling for a stop to Ukraine aid all together over possible diversions.

There are no U.S. boots on the ground in Ukraine, so keeping track of weapons is more of a challenge.