According to Acting Inspector General of the Department of Defense Sean O'Donnell, Ukraine keeps track of the weapons using "hand receipts".

"It is all paper," the official acknowledged, adding that he doubts that the Kiev authorities "have much fidelity" as to where the arms end up. O'Donnell noted that a "lack of effective record-keeping" got in the way of Pentagon investigations in Iraq and Afghanistan.

Meanwhile, NATO officials appear to be "confident that the security was sufficient for the transfer of weapons," O'Donnell said. Interestingly, Russian Defense Minister Sergei Shoigu previously said the arms supplied by the West to Ukraine were ending up on the black market and spreading across West Asia.

Read more: Experts warn arms for Ukraine could have disastrous consequences

O'Donnell's office also intends to look into the spending of Ukraine-related funds, along with intelligence-sharing agreements between Washington and NATO allies, the effectiveness of training of Ukrainian forces, and possible sales of arms in the black market.

Just last Friday, a high-ranking Pentagon representative affirmed during a briefing that the US has been shipping HARM anti-radar missiles (that are used for targeting radar systems) to Ukraine without actually announcing it publicly. The total number of Ukraine-related military contracts signed with the US reached 7,800 valued at a total of around $2.2 billion, with the latest military aid package valued at $2.98 billion. 

A senior US defense official warned, on Tuesday, that hundreds of millions of dollars worth of US weapons sent to Ukraine might “drop into a big black hole, and you have almost no sense of it at all after a short period of time," stressing that the US military supply to Ukraine is "certainly the largest recent supply to a partner country in a conflict." 

However, current US officials and defense analysts agree that the risk is that some of those weapons will end up in the hands of mercenaries.

Moscow has repeatedly denounced the continuous flow of weapons to Ukraine from its Western allies, saying that it adds fuel to the fire. In April, Russia sent a note to NATO member states, condemning their military assistance to Ukraine. In July, Russian Defense Minister Sergei Shoigu said that foreign weapons supplied by the West to Ukraine ended up in illegal markets, not only in Europe but also across West Asia.

Read next: US arms supplies to Kiev may end up in shadow markets of other countries