WASHINGTON — The head of Air Combat Command wants to see the US Air Force build up its inventory of F-35s quicker than planned, but its civilian head signaled Wednesday that it might not be feasible in the current fiscal climate.

Speaking to the press on Tuesday, Gen. Herbert "Hawk" Carlisle said he was concerned about the service's current F-35 buy rate, which hovers in the 40s until fiscal 2021 when that number jumps up to 60. Carlisle stated he would like the Air Force to buy at least 60 aircraft per year in the near term to replace legacy aircraft that is aging out.

But in an interview with Defense News and sister publication Air Force Times, Air Force Secretary Deborah Lee James said boosting the number of active-duty airmen is currently a bigger priority than ramping up the F-35 buy.

"It's all a matter of money. I would love to increase the buy also, but I don't see that as more urgent than staying the course to increase our end strength, for example," she said in an interview Wednesday. "Increasing that end strength is the top thing. We think it's the top thing for all of our senior leaders."

The Air Force is facing a variety of demands, James added, and there simply may not be space in the budget to afford any more F-35s.

"Realistically speaking, given that we've had so much difficulty getting sequestration lifted, I worry about the money. I'm not sure where the money would come from," she said.

Carlisle noted Aug. 2 that increasing the number of F-35s bought would boost economic order quantity, bringing the cost per jet closer to its $85 million target. On the other hand, moving slowly could lead to the Air Force spending more money upgrading fourth-generation airplanes that would otherwise be removed from service.

"I need more [F-35s] sooner to replace legacy airplanes and airplanes that are going to require money to do service life extension and do capability increases if I don't replace the F-35," he said. "So I would like to see the numbers go up to at least 60 if I can. Eighty would be optimum, but given the fiscal constraints that we're in today, 80 would be very, very hard to get to."

The Heritage Foundation, a conservative think tank, released a report on Aug. 4 recommending that the Air Force procure its full 1,763-aircraft program of record throughout the life of the program. Once full-rate production starts, the aircraft will be cost-competitive with other fighters while outclassing them in nearly all capabilities, said John Venable, the senior defense fellow who wrote the report.

"The Air Force is currently deferring the purchase/cashing in on F-35As to pay for other critical needs that have gone unfunded or underfunded by Congress," Venable wrote, referencing the service's fiscal 2017 budget, which reduced its buy of F-35As by five aircraft when compared to the previous year's plan. "That practice needs to end immediately."

But buying more than planned probably isn't in the cards, at least in the near term, said Richard Aboulafia, an analyst with the aerospace and defense expertise firm Teal Group.

Boosting the buy in 2018 is an unlikely prospect because the Air Force would have to take money from other procurement accounts in order to do so, he said. Even then, it has few options. The service cannot make big cuts to its KC-46 tanker program without breaking its fixed-priced contracting agreement. Slashing other procurement programs like the T-X trainer and Joint Surveillance Target Attack Radar System would not yield enough cash to meaningfully increase procurement.

"You're left with two choices," Aboulafia said. "You either grow the top-line [budget] — good luck with that — or you take cash away from the bomber, which doesn't appear to be likely."

Ramping up to a rate of 60 per year in 2021 is still the current plan, James said, but that could change.

"We're working our way through the next five year plan, the [program objective memorandum] that we're putting together right now, and everything in that POM at this point is still up for discussion," she said.

Twitter: @ValerieInsinna