WASHINGTON (Reuters) - U.S. military operations, including the Iraq and Afghanistan wars, have cost $904 billion since 2001 and could top $1.7 trillion by 2018, even with big cuts in overseas troop deployments, a report said on Monday.

A new study released by the nonpartisan Center for Strategic and Budgetary Assessments, or CSBA, said the Iraq conflict’s $687 billion price tag alone now exceeds the cost of every past U.S. war except for World War II, when expenditures are adjusted for inflation.

With another $184 billion in spending for Afghanistan included, the two conflicts surpass the cost of the Vietnam War by about 50 percent, the report said.

CSBA said U.S. military operations have already reached $904 billion since 2001, including the two wars as well as stepped-up military security activities at home and the payout in war-related veterans’ benefits. The estimate includes allocated spending into 2009.

In contrast, a separate Government Accountability Office study released on Monday said Congress has provided the Pentagon with $808 billion for the Bush administration’s global war on terrorism from 2001 through September 30, 2008, including $508 billion for Iraq and $118 billion for Afghanistan, the Philippines and the Horn of Africa.

The CSBA study said U.S. taxpayers could pay another $416 billion to $817 billion over the next decade, even if the combined troop deployment to Iraq and Afghanistan were slashed to between 30,000 and 75,000.

That would bring the cost for both wars to between $1.3 trillion and $1.72 trillion for 2001 through 2018, and even higher when federal borrowing costs are included, CSBA said.

The United States has 143,000 troops in Iraq and 31,000 in Afghanistan. Washington has agreed to withdraw its forces from Iraq by the end of 2011, under a newly signed agreement with the Iraqi government. But U.S. officials are planning to add more than 20,000 forces in Afghanistan within 12 to 18 months.

One reason for the ballooning costs is the Bush administration’s habit of funding the wars through supplemental budget requests, a practice that CSBA said has eroded congressional oversight and weakened the Pentagon’s long-term planning and budgeting processes.

The Bush administration and Congress have also pursued significant tax cuts since 2001 and robust spending increases, rather than following the established approach of funding war costs by combining tax increases with curbs on domestic spending and borrowing.

“The Bush administration has taken a starkly different approach to financing the wars in Iraq and Afghanistan,” the CSBA study said in its executive summary.

In fact, CSBA said war cost projections rise significantly when interest payments on the federal debt are included in the calculations.

Overall costs would reach $1.4 trillion to $1.8 trillion from 2001 through 2018 if borrowing were assumed to cover 10 percent of underlying military operations.

CSBA said war cost projections would climb to between $2 trillion and $2.5 trillion, if all costs were covered by borrowing.