The US military's top officer signalled Thursday that the Pentagon might have to cut one of three planned models of the new F-35 fighter jet, citing budget pressures.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, told lawmakers that while he believed a new fighter was vital he questioned whether a constrained defense budget could fund all three versions of the Joint Strike Fighter, including an aircraft able to take off or land vertically.

"I am concerned about the three variants and whether as we go forward in this fiscal environment, whether we can afford all three," Dempsey told the House Armed Services Committee.

The general said he was ready to "learn more about" the issue and to hear the views of the commandant of the US Marine Corps, General James Amos, a strong advocate of the short-take off, vertical landing (STOVL) version of the plane.

"But I'll tell you, that's something we have to keep an eye on" and three versions of the aircraft "create some fiscal challenges for us."

Under a debt-reduction deal, the Pentagon must cut $450 billion from the defense budget over the next ten years, and the costly F-35 program is seen as a prime target for reductions.

The Joint Strike Fighter, manufactured by Lockheed Martin, is the Pentagon's most expensive weapons program ever. The project has faced chronic delays and cost overruns, with the vertical-takeoff model -- the F-35B -- coming in for the most scrutiny.

Britain, one of the countries participating in the fighter program, has decided to cancel plans to purchase the STOVL model and will instead buy the F-35C designed to land on aircraft carriers.

The Pentagon plans three versions of the plane: the standard F-35A that would replace the Air Force's F-16 fighter, the F-35C designed to land on naval carriers to replace the F-18 jet and the F-35B vertical-takeoff model that would supplant the Harrier aircraft flown by US Marines.

Former defense secretary Robert Gates placed the F-35B on "probation" in January after a spate of technical problems, saying the program had two years "to get it right" or face termination.

Asked at the hearing about the status of the F-35B, the new defense secretary, Leon Panetta, said the probation would allow for further testing to "give us a chance to see how it performs."

"And if it performs well, then obviously it'll be able to make the grade," he said.

The cost of building F-35 jets has mushroomed and defense officials have struggled to keep the price tag under control. Over the past decade, the cost per plane has doubled in real terms, according to the Pentagon.

The program's cost has jumped to about $385 billion, and the price of each is at roughly $103 million in constant dollars or $113 million in fiscal year 2011 dollars.