Afghanistan's Taliban-led insurgency is so heavily funded by foreign donations that U.S. and Afghan officials say it may be impossible to obstruct the group's money supply, the Washington Post reported Sunday.

The Obama administration said the group's single largest source of funding comes from foreign donations -- not drugs like opium -- according to the newspaper.

The CIA reportedly estimates that the Taliban received $106 billion in foreign money last year alone -- making it difficult for intelligence officials to track the terrorist group's money flow.

United Nations' Taliban and Al Qaeda Monitoring Team Coordinator Richard Barrett, told the Washington Post that Taliban supporters have grown much more skillful at masking their donations.

U.S. officials say they suspect that the Pakistani military and intelligence operatives continue to fund the insurgency -- not the Saudi and other Gulf governments, which gave the group substantial financial support in the 1990s -- the newspaper reported.

U.S. and Afghan officials said the diversified funds have allowed the Taliban to expand with relative ease, the Post reported.

David S. Cohen, the Treasury Department's assistant secretary for terrorist financing, told the Post, "We're going after this with a great deal of urgency and a huge amount of effort to even more effectively disrupt the networks that fund the Taliban."

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