The cost of the wars in Iraq and Afghanistan has soared and may now reach $811bn (£445bn), says a report by the Congressional Research Service.
The US is spending more than ever on the conflict in Iraq
It estimates that Congress has appropriated $368bn for the global war on terror, including both conflicts.
It says that if the current spending bill is approved, US war costs will reach $439bn, and it estimates that an extra $371bn may be needed by 2016.
On that basis, the two wars would cost more than the $579bn spent in Vietnam.
The future costing assumes that US troop levels will drop from the 258,000 currently engaged in all operations to 74,000 by 2010.
The rising cost of the war is leading to growing concerns in Congress, where attempts to control the budget deficit have been hindered by the "supplementary" requests received each year for war spending.
The CRS estimates that the US Department of Defense's annual war funding has risen from $73bn in 2004 to $120bn in 2006, with an increase of 17% this year alone.
Monthly war costs have doubled since the fall of Saddam
There have also been concerns that extra non-related appropriations are often tucked inside the war funding bill.
On Thursday Senators deleted funding for a $15m seafood promotion programme that had been tucked away in the current bill.
Earlier, Senators diverted $1.9bn in war funds to pay for increased immigration controls at US borders.
The cost of the war in Iraq has been increasing since US troops have become bogged down in the conflict.
The CRS says the real cost of the conflict in Iraq has risen to $8bn monthly, nearly double the cost in 2003.
It points out that it is difficult to estimate the exact cost of individual operations, such as the Iraq conflict, because the Defense Department does not break down the figures for individual operations.
And it says that the Defense Department has also minimised the cost of the war by not including other costs, including intelligence and the training of Iraqi and Afghan security forces, in its estimates.
Overall, 71% of the total war costs have been spent in Iraq, 21% in Afghanistan, and 7% on increased protection for US forces worldwide.
The main reason for the rapidly escalating costs is increased spending on ammunition, equipment and operational materials such as petrol.
Over $60bn has been spent on procurement, including improved armour, replacement of damaged vehicles, and the building of a more extensive infrastructure to support the troops on the ground.
The CRS says that "if the global war is likely to become the long war as some administration spokesman have suggested, Congress may want to consider requiring that the Department of Defense request a full year's war funds concurrently with its regular budget".
The estimates do not include the costs of reconstruction, which the US originally estimated at $56bn.
A recent report from the General Accounting Office suggested these costs would be much higher, but also said much of the money disbursed so far had been spent on security, not rebuilding.