The U.S. military has only minimal knowledge of - and exercises virtually no control over - the thousands of Afghans it indirectly pays to guard its installations, including "warlords and strongmen linked to murder, kidnapping, bribery" and to the Taliban, Senate investigators said in a blistering report released Thursday.

The bipartisan report, compiled after a year-long investigation, notes that the military has recently launched its own investigations of the situation and has taken some steps to address it. In one of the most significant steps, Gen. David H. Petraeus, the top U.S. and NATO commander in Afghanistan, has issued new contractor guidelines.

Still, the Senate investigation documents a failure to properly vet, train and supervise Afghan security subcontractors, hired by U.S. and other international firms under multimillion-dollar military contracts.

That failure has cost American lives, undermined the U.S. mission and the Afghan government, and "helped play into the hands of the enemy," said Sen. Carl M. Levin (D-Mich.), chairman of the Senate Armed Services Committee.

Some of the Afghan security subcontractors, Levin told reporters Thursday, are "creating the very threat they are hired to combat."

Committee staff reviewed more than 125 Defense Department security contracts dated between 2007 and 2009 and provided a detailed account of two in which subcontractors had direct and well-known ties to the Taliban. The report recounts an instance in which the military raided a Taliban meeting being held at the house of a subcontractor. It also notes instances in which security subcontractors were believed by U.S. military intelligence to be Iranian agents.

According to the U.S. Central Command, the report said, there were more than 112,000 Defense Department contractor personnel in Afghanistan as of April 30. As of May, more than 26,000 armed private security personnel - nearly all of them Afghans - worked for the Pentagon and other U.S. agencies.

Subcontracted Afghans provide perimeter security for U.S. forward operating bases, civilian installations and development projects, as well as for the truck convoys that carry most of the food, fuel, weapons and other supplies for the U.S.-led coalition.

In congressional testimony in December, Defense Secretary Robert M. Gates and Secretary of State Hillary Rodham Clinton acknowledged concern that the United States was indirectly funding warlords and the Taliban. In June, a House subcommittee investigation found that Afghan private security contractors ran a "protection racket" in which militias, some tied to the Taliban, received money to protect supply convoys.

Early this year, Afghan President Hamid Karzai pledged to disband private security contracting firms. This week, Afghanistan's Interior Ministry announced that it had begun disarming those companies that are unlicensed.

In a letter Tuesday to Levin, Gates said the report had helped the Defense Department "understand the nature of the problems associated with contracting in Afghanistan." He said oversight has already been expanded in an effort to "benefit our forces on the ground while not providing aid to our enemies."

The military has been reluctant to remove U.S. troops from combat and other duties to protect the supply convoys. But in the wake of the earlier subcommittee report, it has looked for alternatives, including using Afghan national security forces to guard the trucks. The next step of Karzai's phase-out of the private security firms, U.S. military officials said, will target those providing convoy escorts. Replacement of the "static" security discussed in the Senate committee's report will come at a later date, officials said.