The 'eyes-only' DoD dossier said the U.S. had already spent $664 million in Libya by mid-May - a running cost of $60 million a month since the bombing began in March.
At the current rate of spending, the U.S. will have to shell out at least an extra $274 million till the end of the current 90 day no fly zone extension period - brining total expenditure to a minimum of $938 million.
The news came as donors pledged more than $1.3 billion dollars to help support Libya's main opposition group, after countries backing NATO's military mission there met to prepare for the post-Moammar Gadhafi era.
© The Associated PressCosts: U.S. Defense Secretary Robert Gates speaks during a media conference today. He is under pressure to stump up for the cost of operations in Libya
© EPADefiant: The Libyan people are stronger than NATO's attacks, Libyan leader Muamer Gaddafi, said earlier this week
The leaked document, obtained by The Financial Times, showed the rate of spending is far higher than DoD estimates issued in late march.
Then, a congressional hearing, heard the U.S. had spent about $550 million on Libya, at a rate of about $40m a month.
The soaring cost will only add to the pressure felt by military commanders already grappling with growing budgetary constraints and procurement over spends.
Mr Gates has requested extra funds for Libya operations, but has been rebuffed by the White House.
Speaking in May: 'In the case of Libya, unfortunately, we're fundamentally having to eat that one.'
NATO airstrikes rattled the Libyan capital this morning, with seven thunderous explosions shaking the city.
Concussions from the strikes, in clusters of a few minutes apart, washed over Tripoli from its outskirts. Rebels hold swaths of eastern Libya, although fighting has since become a stalemate even with NATO support.
© EPABombing: Smoke rises in the sky after a NATO air strike in Tripoli. NATO planes bombed the Tripoli compound of Libyan leader Muammar Gaddafi for the second time this week
© The Associated PressPain: A wounded man is brought into the hospital in Misrata. Fighting continues as NATO members started planning for a post Gaddafi future
Italy and France offered a combined $1.02 billion to Libya's Transitional National Council while Kuwait and Qatar promised a combined $280 million to a fund set up to provide transparent assistance to the opposition.
The pledges came as council members appealed for urgent infusions of cash to keep from going broke.
The council is trying to establish an alternative government to take over after Gadhafi.
Preparing for Libya's next phase will require a decision on what fate - exile, prosecution or some third option - should befall the beleagured Gaddafi.
Other issues include the parameters for a ceasefire between rebels and remaining Gaddafi loyalists and the creation of a viable political process that will ensure the democratic aspirations of the Libyan people, according to U.S. officials.
Secretary of State Hillary Clinton, in Abu Dhabi today, disappointed the rebel-affiliated group by saying that while Washington would boost its humanitarian aid to all Libyans by $26.5 million it is not offering any direct aid to the council.
© The Associated PressCornered: Libyan leader Moammar Gadhafi, increasingly cornered under a stunning upturn in NATO airstrikes, lashed back with renewed shelling of the western city of Misrata
© The Associated PressAllies: Secretary of State Hillary Rodham Clinton speaks during a news conference at the Emirates Palace Hotel in Abu Dhabi, United Arab Emirates today
'We are working with our international partners through the U.N. to plan for the inevitable: a post-Gadhafi Libya.'
Fathi Baja, head of political and international affairs for the TNC, said he was encouraged by the growing list of countries that announced they plan to recognize the council as the legitimate government of Libya.
He said Australia announced at the meeting it would do so today while Bulgaria, Greece and Cyprus said they would do so in the coming days.
But he said he was less impressed with the financial commitments, adding that he was hearing more about money in the meeting already promised than new pledges.
He singled out Kuwait, which he said has yet to deliver the $180 million pledge made a month ago.
Although a handful of nations have recognized the council as the legitimate government of Libya, the United States has not.
That has delayed efforts to free up some of the more than $30 billion in Libyan assets that have been frozen in U.S. accounts.
The U.S. said on Wednesday that the first shipment of Libyan oil sold by the council had been delivered to an American refinery and Clinton encouraged other nations to make similar purchases to help the Libyan people.
I don't quite follow their math, but that could just be me. The war started in Mid feb if I remember correctly and it says that by mid May they had already spent 664 million. That makes it 664 million for three months of war or 221 million per month or just over 10 million per day. It that a deliberate mistake by the media to downplay the costs?
Regarding the donors, Italy and France, I guess this is the price the taxpayers in those countries will have to pay so that Italian and French corporations can profit in the aftermath, by (looting) sharing the oil contracts and other resources.