Estimates of the cost of the next generation fighter jet are unbelievable, a senior Pentagon official said Thursday as he insisted the program's price tag must be reined in for the military to purchase more than 2,400 aircraft for the Air Force, Navy and Marine Corps.

Ashton Carter, who oversees acquisition, said there are no alternatives to the F-35 Joint Strike Fighter, a stealthy aircraft manufactured by Lockheed Martin, and the Defense Department was working to make the program affordable. The jet fighter's costs have increased 26 percent while its schedule has slipped five years due to design changes, problems with software development and technical problems.

Democrats and Republicans on the Senate Armed Services Committee expressed frustration with the program, the Pentagon's largest acquisition effort. Ten years into the program, the cost has jumped from $233 billion to $385 billion. Recent estimates say the entire program could exceed $1 trillion over 50 years.

"If we live the estimates, we can't afford to pay that much," Carter told the Senate Armed Services Committee. "I don't think we have to live those estimates and that's our objective is to make sure that those estimates don't come true and that we do have an affordable program."

Exasperated members of the committee pressed the Pentagon officials and a Lockheed Martin executive on what steps they were taking to get the program in line, explaining that Washington's push for fiscal austerity would not allow excessive spending on the aircraft.

"We cannot sacrifice other important acquisitions in the DOD investment portfolio to pay for this capability," said Sen. Carl Levin, D-Mich., chairman of the committee.

The panel's top Republican, Sen. John McCain of Arizona, put it more bluntly.

"No program should expect to be continued with that kind of track record, especially in our current fiscal climate," McCain said.

Carter said the Pentagon would have a better idea in the next few months on the cost of the Joint Strike Fighter as they were closely scrutinizing the program. He stressed that there was no reliable alternative and the military had to have the plane, which eight other countries also are planning to purchase.

Michael Sullivan, an analyst with the Government Accountability Office, told the committee that the aircraft program "has not fully demonstrated that the aircraft design is stable, manufacturing processes are mature and the system is reliable."

But Carter rejected any suggestions about changing the design.

"The last place I want to go is dumb down the aircraft," Carter said.

Tom Burbage, executive vice president and general manager at Lockheed Martin, said he understood the committee's concern on the schedule and cost and said the company was "committed to drive down costs in the face of challenging fiscal realities."

McCain pressed Burbage on what were the "handsome returns to shareholders" of Lockheed Martin compared to the nonexistent returns for the American taxpayer. Burbage said he could not provide the numbers.