American military personnel are now in Ukraine to help keep track of the billions of dollars’ worth of weapons and equipment the United States has sent since the start of the Russian invasion, a senior U.S. defense official and senior U.S. military official said. 

Led by Brig. Gen. Garrick Harmon, the U.S. defense attaché to Ukraine, the inspections have already begun with the help of the Office of Defense Cooperation personnel who have returned to the U.S. Embassy in Ukraine, the officials said. The U.S. had conducted similar checks on aid prior to the war, but they stopped for months after Russia invaded on Feb. 24.

“There have been several of these inspections,” according to the senior defense official, who declined to give details on the locations of the on-site inspections. The Ukrainians have been “very transparent” and are supporting the inspections, the official added.

NBC News has not confirmed how many members of the U.S. military are in Ukraine to conduct the inspections, how many inspections they’ve completed or when the program restarted.

These inspectors in Ukraine appear to be some of the first members of the U.S. military to re-enter the Eastern European country since the start of the war, outside of military guards posted at the U.S. Embassy in Kyiv, the capital. The Pentagon ordered the departure of U.S. troops in Ukraine on Feb. 14 — 10 days before the invasion — as the crisis escalated.

The inspections come after Russia and some Republicans in Congress have alleged that weapons and military equipment sent to Ukraine may have ended up on the black market. Serving as a rebuttal to these concerns, the Biden administration released a plan last week that would aim to keep closer tabs on the aid it has sent.

The U.S. has not seen any evidence of weapons being diverted to a black market or used for anything other than their original purpose, the defense official said, but the Pentagon and State Department remain aware of those risks and are taking efforts to prevent it.

“Thus far, intense internal demand for use on the battlefield by Ukrainian military and security forces within Ukraine is assessed to be impeding black-market proliferation of small arms and guided infantry weapons,” the State Department document states.

There is some concern about Russia's ability to capture U.S. weapon systems, however. The administration said in its plan that pro-Russian forces have been "the main vector of diversion so far and could result in onward transfer" of Ukrainian weapons and "donated materials."

"Russia probably will also use these weapons to develop countermeasures, propaganda, or to conduct false-flag operations," the State Department document said.

The plan notes two specific types of weapon that have become particularly popular in Ukraine during the war as areas of concern. The document mentions portable air defense systems, such as the U.S.-provided Stinger anti-aircraft missiles, and the anti-tank/all-purpose tactical guided missiles, which would include the Javelin missiles sent by the U.S.

To rebut the potential loss of weapons, the administration has drawn up a number of actions it intends to take. Some are to begin this year and others would start in 2023 and 2024, though few details have been released.

The intention is for Kyiv to beef up border security for the illegal movement of weapons, further investigate suspected weapons traffickers, increase training for guards and inspectors, and ensure the destruction of found weapons and ammunition. U.S. Embassy officials in Kyiv, meanwhile, would aid Ukraine in its weapon tracking. The plan acknowledges, however, "that the chaotic nature of combat can make this difficult."

“The Ukrainian government has committed to appropriately safeguarding and accounting for transferred defense equipment,” State Department spokesperson Ned Price said in a statement last week. “As in any conflict, we remain vigilant to the possibility that criminal and non-state actors may attempt to illicitly acquire weapons from sources in Ukraine, including members of the Russian military, during or following the conflict.”

Facing similar allegations in July, Ukrainian Defense Minister Oleksii Reznikov denied that donated weapons were making their way to the black market. While he acknowledged that Ukraine could do more to keep closer tabs on the weapons flowing into the country, he said that the accusations were unfounded and part of a Russian disinformation campaign.

“We need to survive," Reznikov told the Financial Times. "We have no reason to smuggle arms out of Ukraine."

The danger remains, however, and politicians such as Republican Sens. John Kennedy of Louisiana and Rand Paul of Kentucky have stated that greater oversight was needed.

Others have also raised concerns about the sudden increase in the number of weapons in Europe.

Shortly after the Reznikov interview, the European law enforcement agency Europol said in a statement "that the proliferation of firearms and explosives in Ukraine could lead to an increase in" weapons trafficking. "This threat might even be higher once the conflict has ended."

Courtney Kube is a correspondent covering national security and the military for the NBC News Investigative Unit.

Phil McCausland is an NBC News reporter.