Men inspect a site hit by what activists said was shelling by forces loyal to Syria's President Bashar al-Assad, in the Duma neigbourhood of Damascus September 4, 2013.
Defense Secretary Chuck Hagel told lawmakers a limited military strike to deter Syria from using chemical weapons would likely cost tens of millions of dollars, but if past experience is a guide, the number could be substantially higher than that.
It is not uncommon for U.S. forces to open an assault by launching scores of Tomahawk missiles costing over $1 million apiece and dropping bombs from radar-evading B-2 planes that fly 18 hours each way from their base at a cost of $60,000 an hour.
"I was surprised when I heard him (Hagel) say tens of millions of dollars. That's low-balling it," said Todd Harrison, a defense budget analyst at the Center for Strategic and Budgetary Assessments.
He said the defense secretary might have been thinking of what the Pentagon would have to spend during the remainder of the 2013 fiscal year, which ends on September 30.
Most of the cost of an action against Syria would be for replacing munitions that were used, funds that would not be required until after the 2014 fiscal year begins on October 1.
The Pentagon probably would pay for the munitions with a supplemental war-funding request to Congress, which would not be subject to current budget spending caps, Harrison said.
"If you include the replacement costs of munitions, it (an operation against Syria) could cost half a billion, up to a billion dollars depending on the number of targets they go after," he said.
Tomahawk cruise missiles, which have a thousand-mile (1,600-km) range, can loiter on station and change their targets in flight, are expected to be the main weapon if President Barack Obama orders a limited strike to punish Syria over its suspected use of chemical weapons.
The missiles cost $1.2 million to $1.5 million apiece.
The U.S. Navy fired 221 Tomahawks in operations against Libyan leader Muammar Gaddafi in 2011, nearly half of them - 110 - in an opening salvo against 22 Libyan military targets, including air defenses, communications and command structures.
If U.S. forces used a similar number of missiles to hit Syrian targets related to chemical weapons use by President Bashar al-Assad's forces, the cost would top $100 million.
Admiral Jonathan Greenert, the top U.S. Navy officer, said on Thursday that operations so far haven't required unexpected spending.
U.S. warships in the region were all overseas as part of regular operations. The Navy has four guided missile destroyers in the eastern Mediterranean and the aircraft carrier USS Nimitz with its supporting vessels in the Red Sea.
"The ships ... were all forward deployed, so we haven't surged anybody over for this ... specifically," Greenert told the American Enterprise Institute think tank.
But the Nimitz was scheduled to rotate home after being replaced in the Arabian Sea by the USS Harry S Truman, so if it is held much longer in the region it could result in an unplanned hit to the budget, he said. A supplemental budget request might be needed to pay for an operation, he said.
Greenert said it costs about $25 million a week for a carrier strike group in routine operations. If the carrier was used in military operations, the cost would rise to $40 million a week as a result of longer flight hours for its planes.
While Hagel estimated the cost of a Syrian operation at tens of millions of dollars at a House of Representatives hearing on Wednesday, Pentagon officials have declined to elaborate on his remarks or discuss costs further.
"I'm not going to get into specific numbers because I don't want to suggest that we have a precise picture of the military operation that would be conducted," Pentagon spokesman George Little told a briefing on Thursday.
He also declined to speculate on how the Pentagon would pay for such an operation at a time of tight budgets.
"This is in the national security interests of the United States," Little said. "When something is that important, we'll find a way to pay for it."