WASHINGTON — The federal government sued KBR Inc., the largest contractor in Iraq, on Thursday over what prosecutors say were improper charges to the Army for private security services there.

The Houston-based KBR is a former subsidiary of Halliburton. It recently won a new contract potentially worth more than $2 billion for support work in Iraq.

The lawsuit, filed in federal court in Washington, charged that KBR and 33 of its subcontractors used private armed security at various times from 2003 to 2006. The suit claimed KBR knew under the terms of its contract the company could not bill the U.S. government for such services but did so anyway.

While the lawsuit is a contractual dispute, the case highlights what became a confusing problem in the U.S. occupation of Iraq: What authority did private contractors have to carry weapons and use force in the unstable country?

KBR reported 2009 revenues of $4.8 billion from work under the Logistics Civil Augmentation Program, according to the independent Commission on Wartime Contracting in Iraq and Afghanistan.

But under terms of that program, prosecutors said, KBR was to provide logistical support, such as food services, transportation, laundry and mail for U.S. military operations in Iraq. Security protection was to be provided by the U.S. military under terms of the contract.

The government’s court papers did not say how much money the improper billing may have cost the government.

The suit charged that KBR violated terms of the contract by failing to get Army authorization to arm subcontractors and by using private security contractors who were not registered with the Iraq Ministry of the Interior.

“Defense contractors cannot ignore their contractual obligations to the military and pass along improper charges to the United States,” said Tony West, head of the Justice Department’s civil division.

A spokeswoman for the company did not immediately comment on the lawsuit.

The case grew out of a 2006 initiative aimed at cracking down on fraud in government contracting.

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Associated Press writer Kimberly Hefling contributed to this report.

Cheyenne Mountain High School graduate Steve Johnson is a reserve forward on the Duke men’s basketball team, which will play West Virginia on Saturday in the Final Four in Indianapolis. A redshirt junior, Johnson has seven points and four rebounds in 39 minutes in 21 career games for Duke, gunning for its first national title since 2001. Photo by THE ASSOCIATED PRESS

Jon Gardiner