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.
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.