These unanticipated costs come at a time when the Pentagon is putting pressure on Capitol Hill to pass a fiscal 2011 defense budget. Continuing to operate under a stopgap continuing resolution through September, senior defense officials argue, would amount to a $23 billion cut to the military's request for the current fiscal year, which began Oct. 1. The Pentagon wants $708.3 billion for this year, including $159.3 billion for the wars in Iraq and Afghanistan.

Still, the Pentagon has the money in its budget to cover unexpected contingencies and can also use fourth-quarter dollars to cover the costs of operations now. "They're very used to doing this operation where they borrow from Peter to pay Paul," said Gordon Adams, the White House Office of Management and Budget's associate director for national security during the Clinton administration.

Indeed, former Pentagon comptroller Dov Zakheim estimated that the Defense Department would only need to send a request for supplemental funding to Capitol Hill if the U.S. military's share of operations expenses for Libya topped $1 billion. Such a request would likely be met with mixed reactions in a Congress focused on deficit reduction. And, while many key lawmakers have been agitating for action in Libya, others have been more reluctant and have urged the Obama administration send them a declaration of war.

Senate Foreign Relations ranking member Richard Lugar, R-Ind., says Congress should have had the opportunity to weigh in on what he said will be "a very expensive operation, even in a limited way."

"It's a strange time in which almost all of our congressional days are spent talking about budget, deficits, outrageous problems," Lugar said Sunday on CBS's Face the Nation. "And yet [at the] same time, all of this passes."

For the U.S. military, the highest costs come in the form of pricey munitions, fuel for aircraft and combat pay for deployed troops - all factors that will pile up each day U.S. forces remain at the helm of the operation.

On the first day of strikes alone, U.S.-led forces launched from ships stationed off the Libyan coast 112 long-range Tomahawk cruise missiles, which cost in the range of $1 million to $1.5 million apiece. That is $112 million to $168 million for the first day's strike in missiles alone. The military will eventually refill its stockpile though those costs could be pushed off for months or more.

Pentagon budget watchers said the deployment of guided missile destroyers and submarines would not put a major dent in the Pentagon's accounts because the ships were already deployed to the region. But the U.S. military on Sunday tapped its B-2 bombers, and F-15 and F-16 fighter jets to strike a number of targets in and around Tripoli, which will undoubtedly force an immediate uptick in the military's operations and maintenance expenditures, including fuel costs.