The war in Libya is costing the US Air Force $US4 million a day but the cost will likely drop now American fighter jets have pulled out of the operation, the air force secretary said.

As of Tuesday morning, "we're probably (at) about $US75 million for the cost of the operation thus far," said air force secretary Michael Donley, the top civilian overseeing the service.

"And our best estimate was about $US4 million a day."

Speaking to defence reporters in Washington, Mr Donley said the price tag "will probably be adjusted here as the US participation in strike operations has now gone away".

But he could not say how much the cost of the operation might be reduced after the withdrawal of US combat aircraft.

About 50 warplanes - including fighter jets and ground-attack aircraft - were deployed for combat flights over Libya in the past few days, while the air force has contributed 39 "support aircraft" for refuelling, surveillance and other tasks, according to Mr Donley.

He said the strike aircraft would return to European bases and remain on standby in case NATO requested help from Washington in the air campaign, launched March 19.

US combat sorties ended at 2200 GMT on Monday as NATO allies take over strike missions, under a UN-mandate to enforce a no-fly zone and protect civilians from the Libyan regime's forces.

The total cost of the operation for the entire US military was estimated at $US500 million on March 28.

For relief efforts in Japan after that country's earthquake and tsunami, the air force had spent about $US40 million evacuating 5,000-6,000 Americans and about $US8 million-$US9 million on humanitarian assistance, including food supplies, Mr Donley said.

The air campaign in Libya and the disaster relief operation in Japan have aggravated mounting financial pressures on the Pentagon, as Congress has yet to approve spending to cover the current fiscal year for the military or the rest of the federal government.

- AFP