The military intervention in Libya has cost the United States “about $550 million” in extra spending so far, a Defense Department spokeswoman said Tuesday, providing the first official estimate of the mission’s price tag.

Future costs involved in maintaining a U.N.-mandated no-fly zone in Libya and protecting civilians remain “highly uncertain,” said the spokeswoman, Cmdr. Kathleen Kesler.

But Kesler said those costs are expected to run about $40 million over the next three weeks as the U.S. military scales back its activity and NATO forces take the lead. Thereafter, the Pentagon estimates that its added costs for the Libyan operation will total about $40 million a month.

Almost 60 percent of the total cost has been for munitions — the most expensive of which are Tomahawk missiles, which cost more than $1 million each to replace. The added higher operating costs of aircraft, ships and submarines are relatively small since they would normally be in service.

Unforeseen military operations typically require supplemental appropriations because they fall outside the core Pentagon budget.

President Obama has said the Libya mission could be paid for with money already appropriated for the Pentagon, but Republican lawmakers have pressed the president on whether supplemental funding will be requested from Congress.