Russia has been capturing some of the US and NATO-provided weapons and equipment left on the battlefield in Ukraine and sending them to Iran, where the US believes Tehran will try to reverse-engineer the systems, four sources familiar with the matter told CNN.

Over the last year, US, NATO and other Western officials have seen several instances of Russian forces seizing smaller, shoulder-fired weapons equipment including Javelin anti-tank and Stinger anti-aircraft systems that Ukrainian forces have at times been forced to leave behind on the battlefield, the sources told CNN.

In many of those cases, Russia has then flown the equipment to Iran to dismantle and analyze, likely so the Iranian military can attempt to make their own version of the weapons, sources said.

Russia believes that continuing to provide captured Western weapons to Iran will incentivize Tehran to maintain its support for Russia’s war in Ukraine, the sources said.

US officials don’t believe that the issue is widespread or systematic, and the Ukrainian military has made it a habit since the beginning of the war to report to the Pentagon any losses of US-provided equipment to Russian forces, officials said.

Still, US officials acknowledge that the issue is difficult to track.

It’s not clear if Iran has successfully reverse-engineered any US weapons taken in Ukraine, but Tehran has proven highly adept at developing weapons systems based on US equipment seized in the past.

A key weapon in Iran’s inventory, the Toophan anti-tank guided missile, was reverse engineered from the American BGM-71 TOW missile in the 1970s.

The Iranians also intercepted a US-made drone in 2011, a Lockheed Martin RQ-170 “Sentinel”, and reverse-engineered it to create a new drone that crossed into Israeli airspace in 2018 before being shot down.

“Iran has demonstrated the capability to reverse-engineer US weapons in the past,” said Jonathan Lord, a senior fellow and director of the Middle East security program at the Center for a New American Security.

“They reverse-engineered the TOW anti-tank guided missile, creating a near-perfect replica they called the Toophan, and have since proliferated it to the Houthis and Hezbollah. Iran could do the same with a Stinger, which could threaten both civil and military aviation throughout the region."

He added that: "A reverse-engineered Javelin could be used by Hamas or Hezbollah to threaten an Israeli Merkava tank. In the hands of Iran’s proxies, these weapons pose a real threat to Israel’s conventional military forces.”

The coordination is yet another example of Moscow’s growing defense partnership with Tehran, which has intensified over the last year as Russia has become increasingly desperate for external military support for its war against Ukraine.

The partnership is not only further destabilizing Ukraine, but it could also threaten Iran’s neighbors in the Middle East, National Security Council spokesperson John Kirby said last month.

CNN has reached out to the Russian embassy in Washington and the Iranian UN mission for comment.

US has been warning about threats posed by Iran

Senior US military officials, including Joint Chiefs Chairman Gen. Mark Milley and Secretary of Defense Lloyd Austin, were both in the Middle East this month for discussions with their counterparts there that centered around the threats posed by Iran, the Pentagon said.

“Over the past year, Russia’s military cooperation with Iran has deepened. and that poses serious challenges for this region and for the safety of your citizens,” Austin said at a press conference alongside Israeli Defense Minister Yoav Gallant in Tel Aviv on Thursday.

“Iran is gaining important battlefield expertise and experience in Ukraine that will eventually transfer to its dangerous proxies in the Middle East,” Austin said.

“In return for Iranian support in Ukraine, Russia has been offering Iran unprecedented defense cooperation, including on missiles and air defense.”

Huge, unanticipated losses of equipment and harsh Western sanctions have made it difficult for Russia to continue producing the weapons and ammunition it needs to maintain its offensive in Ukraine.

As a result, Russia has asked for and received hundreds of drones from Iran, as well as artillery and tank rounds, that Russian forces have used to devastating effect against Ukrainian civilians and infrastructure.

In exchange, Iran has sought billions of dollars worth of military equipment from Russia, according to the White House, including fighter jets, radar systems and helicopters.

The Pentagon late last year expanded its efforts to track US weapons provided to Ukraine, including through on-site inspections conducted by US military personnel stationed at the US embassy in Kyiv.

Undersecretary of defense for policy Colin Kahl told lawmakers earlier this year that the US has seen instances of the Russians capturing some US-provided systems on the battlefield, but not in large numbers.

“What we’re not seeing is any evidence of significant diversion,” Kahl said.

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

Asked about this reporting, a defense official referred CNN to Kahl’s comments.

The State Department also said in an October fact sheet that “pro-Russian forces’ capture of Ukrainian weapons, including donated materiel, has been the main vector of diversion so far and could result in onward transfers.”

The department also specifically cited man-portable air defense systems – a category that includes Stingers – and anti-tank missiles as “particularly sensitive and advanced conventional weapons” that, if diverted, could hinder regional security.