At issue is the World Bank-administered Afghanistan Reconstruction Trust Fund (ARTF) that comprises 34 donor nations and agencies and has contributed more than $10 billion to Afghanistan's reconstruction effort.

The largest contributor is the United States, which has paid in more than $3 billion since 2002.

According to a new audit by the office of the Special Inspector General for Afghanistan Reconstruction, the World Bank is unable to accurately evaluate the impact of the funds, or monitor where the money goes.

"Billions of dollars of donor funds contributed to the ARTF are at risk due to continued limitations on, and lack of transparency into, monitoring and accounting of ARTF funding by the World Bank and the government of Afghanistan," SIGAR said in a statement.

As an example, SIGAR said that when the World Bank approves reimbursements for Afghan workers, such as teachers, it does not require a third party monitor to check that the salary recipients actually exist -- despite acknowledging a risk of "undetected ghost workers."

The report also found that donors are unable to specify where they want funds to go, with the result that some monies are being spent in regions controlled by the Taliban or other insurgent groups.

The US Agency for International Development (USAID), which provides the US contribution to the reconstruction fund, told SIGAR it is "no longer preferencing funds by geographic location," the report noted.

SIGAR added that once the US or any other donor provides its contributions to the fund, "neither the World Bank nor USAID can account for how those funds are specifically spent."

In its written response to the audit, the World Bank said it laid "out an opportunity for the government of Afghanistan, ARTF donors and the World Bank to strengthen the focus on results and accountability."

The United States has spent 16.5 years and about $1 trillion overall in Afghanistan, including warfighting and aid, yet the Taliban and other insurgent groups still control or are pushing for influence in large swaths of the country.