The leaders of the Senate Armed Services Committee suggested on Monday that Defense Secretary Leon Panetta rushed a decision to develop the Marine Corps version of the next-generation strike fighter jet despite new technical problems with the troubled program.

In a letter to the Pentagon chief, Sens. Carl Levin, the committee chairman, and John McCain, the panel's top Republican, questioned whether the F-35B had met the criteria to warrant an end to its probation. Last January, then Defense Secretary Robert Gates had put the Marines' version of the aircraft on a two-year probation because of "significant testing problems."

Panetta ended the probation late last month.

Levin, D-Mich., and McCain, R-Ariz., wrote that the program "has enjoyed some success over the last few months, after several years of having fallen short." But they said "more problems with the F-35B's structure and propulsion, potentially as serious as those that were originally identified a year ago, have been found. This is salient where the F-35B has completed only 20 percent of its developmental test plan to date. Your decision, therefore, appears at least premature."

The two senators sent a list of 14 questions about the program's development and the advice Panetta received before making his decision.

In announcing his decision last month, Panetta said the military was committed to developing the aircraft — which can take off from shorter runways and land vertically — but that the program was "not out of the woods yet."

The F-35 is the Pentagon's most expensive weapons program, and it has been troubled by schedule delays and cost overruns. Ten years in, the total F-35 program cost has jumped from $233 billion to an estimated $385 billion. Recent estimates suggest the entire program could exceed $1 trillion over 50 years.

The developer of the aircraft, Lockheed Martin Aeronautics Co., is building three versions of the F-35 — one each for the Navy, Air Force and Marine Corps. The plane would replace Cold War-era aircraft such as the Air Force F-16 fighter and the Navy's F/A-18 Hornet.

In outlining next year's defense budget, Panetta said the administration would slow the purchase of the F-35, a step Congress would have to approve.

In their letter, Levin and McCain expressed frustration with the Pentagon's consultation with the committee about the aircraft and complained that they learned of Panetta's decision on ending probation from media reports.