The U.S. military said Tuesday it has issued new orders to private contractors in Iraq to crack down on violations of human trafficking laws involving laborers brought in from around the world to work on American bases and other sites.

An inspection completed in late March uncovered evidence that it was widespread practice among firms providing services to the military to take away their workers' passports to keep them in place, military spokesman Lt. Col. Barry Johnson said.

Hundreds of thousands of foreign laborers — many from South Asia — are employed by contractors working in U.S. bases and elsewhere in Iraq as cooks, food servers, janitors, construction workers and other menial jobs.

Human rights groups have reported complaints by some workers that they were tricked into coming into Iraq, paying recruiters in their home countries fees for jobs said to be in the Gulf, then forced to go to Iraq after their passports were taken.

The groups have also reported complaints of withheld pay and overtime and unsuitable working and housing conditions provided by contractors and subcontractors for their workers.

The U.S. military inspection found that employers violated U.S. law by withholding passports from their workers to prevent them from jumping to other employers, Johnson said in a statement to The Associated Press.

"The rights to freedom of movement and quality living standards are serious issues; (the U.S. military in Iraq) takes a zero tolerance approach to any violation," he said.

The military has ordered contractors and subcontrators at all levels to return worker passports no later than May 5 and write into contracts restrictions on how long employers can hold travel documents, he said.

Future U.S. military contracts with private firms will spell out "measurable, enforceable standards for living conditions" and require a minimum 50-square-foot personal living space for each worker.

Employers will also be required to provide workers a copy of their contract — a response to complaints that employees are intentionally kept in the dark about terms of their work.

Johnson did not say whether the military was considering prosecuting any contractors or subcontractors for violations the inspection uncovered.

The new orders were issued by Gen. George W. Casey Jr., the top commander in Iraq, in memos on April 4 and 19, according to the Chicago Tribune, which first reported on the new regulations Tuesday.

The inspections confirmed a wide range of violations by contractors — including deceptive hiring practices, excessive fees by recruiters and substandard living conditions for workers in Iraq, the Tribune reported.

Casey threatened possible termination of contracts or blacklisting from future military contracts against violators of the new rules, it said.

The biggest contractor in Iraq — KBR, a subsidiary of Halliburton, the Houston-based firm once headed by Vice President Dick Cheney — has a $8.3 billion deal with the military to provide support for soldiers, including meals, housing, laundry and other services.

It has taken on numerous subcontractors to carry out much of the work, including the recruitment of employees.

A Halliburton spokeswoman, Melissa Norcross, said KBR "fully supports the Department of Defense's efforts to ensure that all contractor and subcontractor personnel working for the U.S. government be treated in a fair and humanitarian manner."

She said in a statement e-mailed to AP that KBR operates under a code of business conduct that it expects subcontractors to follow. "KBR believes that all personnel should be treated with dignity and respect," she said.