Tens of millions of dollars of U.S. taxpayer funds are indirectly being paid to Afghan warlords to secure convoys carrying supplies to U.S. troops in Afghanistan, according to a congressional investigation on Tuesday.

A soldier of Afghanistan army trains at a training station in Kabul, capital of Afghanistan, June 14, 2010. NATO-led International Security Assistance Force (ISAF) is scheduled to launch a large scale military operation in south Afghan Kandahar Province. [Sarosh/Xinhua]

A lengthy report released late Monday said eight Afghan-based private contractors working with the U.S. Defense Department through a 2.1 billion dollars transportation contract are paying several thousand dollars per truck for guards.

The contract covers at least 70 percent of all goods and services used by U.S. forces.

"This arrangement has fueled a vast protection racket run by a shadowy network of warlords, strongmen, commanders, corrupt Afghan officials, and perhaps others," Representative John Tierney, chairman of a House of Representatives national security subcommittee, said in a statement.

Tierney, a Democrat, said the system "runs afoul" of the Defense Department's own rules and may be undermining the U.S. strategic effort in Afghanistan.