WASHINGTON – The cost of building the F-35 fighter jet, set to replace a large part of the US warplane fleet, is "unaffordable" in its current version and must be reviewed, the Pentagon's top acquisition official said Thursday.

"Over the lifetime of this program, the decade or so, the per-aircraft cost of the 2,443 aircraft we want has doubled in real terms," said Ashton Carter, the under secretary of defense for acquisition, technology and logistics.

"That's our forecast for how much the aircraft's going to cost.

"Said differently, that's what it's going to cost if we keep doing what we're doing. And that's unacceptable. It's unaffordable at that rate."

The cost of the plane has jumped to $385 billion, about $103 million per plane in constant dollars or $113 million in fiscal year 2011 dollars, said Christine Fox, the Defense Department's director of cost assessment and program evaluation.

Republican Senator John McCain called the figure "truly troubling," considering the original price was $69 million per airplane.

"The facts regarding this program are truly troubling," said McCain. "No program should expect to be continued with that kind of track record, especially in our current fiscal climate," said McCain.

"It seems to me we have to start at least considering alternatives"

The F-35 or Joint Strike Fighter (JSF), made by Lockheed Martin, is the Pentagon's most costly weapons program.

An additional appropriation of $4 billion brings the cost of development of the plane to $51 billion, "dismaying" figures," said Michigan Senator Carl Levin, who chairs the Senate Armed Services Committee.

"These are dismaying; indeed, they are disturbing numbers in costs to us and to the taxpayers of the United States," Levin said.

Carter attributed the cost overruns and delays to the plane's complexities and its radar-evading equipment, as well as the short-takeoff and vertical takeoff versions. And he said the culture of easy money at the Pentagon since 9/11 was also to blame.

"In the decade of ever-increasing defense budgets, which we just enjoyed, it was always possible for our managers, when they ran into a technical problem or a difficult choice, to reach for more money," said Carter. "And the money was available in the decade after 9/11."