WASHINGTON -- The Afghan National Army may have broken the U.S.-led economic embargo against Iran by using American aid to buy Iranian fuel for its military vehicles, generators and cooking processes, according to a military audit and experts on the region.

Recent safeguards installed to stop the possibility of the practice might not be enough, according to the audit, which came out in January.

While there is no direct evidence that the Afghan army actually purchased Iranian oil with American tax dollars, the report by the Special Inspector General for Afghanistan Reconstruction describes the possibility as a cause for concern.

Between a third and half of all fuel bought by Afghanistan comes from Iran, the audit stated, citing figures compiled by the Department of State. Another quarter of its supply comes from Turkmenistan, and is commonly mixed with Iranian fuel.

The report notes that the U.S.-funded fuel purchasing program between 2007 and 2012 provided at least $1.1 billion to the Afghan army. But the money has been so poorly tracked, the audit notes, that it is impossible to determine the origin of the fuel.

“The fact that the United States paid for the acquisition and delivery of imported fuel for the Afghan National Security Forces … raises concerns that U.S. funds could have been used to pay for imports of fuel potentially in violation of U.S. economic sanctions against Iran,” the audit stated.

In a related development, the U.S. Government Accountability Office on Monday issued a wide-ranging report on Afghanistan funding that noted, "Persistent corruption in Afghanistan undermines security and the people’s belief in the government, as well as effective accountability of U.S. funds provided directly to the Afghan government."

Afghanistan gets around international sanction requirements on Iranian goods for reasons of economic necessity, and – on a more practical level – because the countries share a long border, and goods into Afghanistan increasingly pass through Iranian ports.

Jamie Graybeal, deputy public affairs officer for the International Security Assistance Force in Afghanistan, said the Combined Security Transition Command responsible for overseeing the fuel program “recognizes and appreciates” the concerns of the report.

“Despite actions taken by the Department of Defense to prevent the purchase of Iranian fuel with U.S. funds, risk remains for the potential violation of U.S. economic sanctions,” he said by email.

He added that the command has conducted a review, but found no support for allegations that “DOD funds were used to support the Iranian petroleum industry, but we will continue to work closely with (the special inspector general) on this and other issues.”

The report, however, raises questions about whether the primary means to address this concern – a requirement that all Afghan companies selling fuel to the Afghan National Security Forces certify that their fuel does not come from Iran – lacks the investigative teeth to ensure that it occurs.

Drew Pusateri, a spokesman for Sen. Claire McCaskill, D-Mo., who leads the Senate Subcommittee on Contracting Oversight, said by email that the senator was “concerned by the lack of oversight and accountability detailed in the report and will be reviewing it with her policy staff in the coming days.”