BAGHDAD — A U.S. government study released Monday found that Iraq has a budget surplus of $51 billion, with $11.8 billion that is readily available for spending on its security forces.

The study by the Government Accountability Office, the investigative arm of Congress, comes as the Senate prepares to debate a $2 billion funding request from the White House for the Iraqi security forces.

“”Billions of dollars that Iraq has budgeted for security have gone unused. As U.S. troops withdraw, the Iraqi government must take a larger role in providing security throughout the country,”” the report said.

The Pentagon contested the findings, saying that the dollar figures were incorrect and that Iraq needed its cash reserves to meet expected budget deficits.

The U.S. military had previously protested a recommendation from Sen. Carl Levin, a Michigan Democrat and chairman of the Senate Armed Services Committee, that $1 billion should be slashed from the White House’s proposed $2 billion allotment for 2011. Levin argued that Iraq has enough oil revenue to pay more for defense.

The former U.S. military commander for Iraq, Gen. Ray T. Odierno, has argued that the full funding is necessary for building up Iraq’s border security forces and its army logistics. In July, he told Washington reporters that Iraq would not see a substantial return from foreign investment in its oil fields for three or four years.

The House of Representatives has already approved the $2 billion request.

The GAO said it had audited Iraqi funds, and criticized the Iraqi government for not spending enough. It found that Iraq had failed to spend between $2.5 billion and $5.2 billion that it had budgeted for the Iraqi army and police between 2005 and 2009.

It added that $11.8 billion in uncommitted surplus funds should allow Baghdad to spend more despite concerns about budget deficits. “”In light of these resources, Iraq has the potential to further contribute toward its security needs, even as it addresses other competing priorities,”” the report said.

The Defense Department disputed the GAO’s findings in a letter attached to the study commenting on a preliminary draft. “”We believe that the current overall GAO message that Iraq currently has significant cash reserves that would allow them to pay more of their security costs now and in 2011 — is inaccurate and not supported by the financial data,”” said Colin Kahl, deputy assistant Defense secretary for the Middle East.

Kahl argued that the full White House funding request was important for maintaining long-term ties between Iraq and the United States as remaining U.S. troops prepare to leave Iraq at the end of 2011. More than 49,000 American troops remain in Iraq to support and train Iraqi security forces after President Barack Obama declared an end to U.S. combat operations at the end of August.