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Even under current cuts, “we’re going to have a smaller force,” Mr. Panetta said.

Mr. Smith agreed that going to “sequestration,” as automatic cuts are called, would be “frankly insane.” He called on Republicans to end their no-new-taxes stand in deficit-reduction talks.

“I believe that we need to put everything on the table in trying to deal with our budget deficit,” he said. “I am so concerned about cuts, not just in [the Department of Defense] but in other parts of our budget, that I’m willing to say that we need more revenue, that we can’t take that piece off the table.”

Gen. Dempsey signaled that the Pentagon’s largest procurement program, the F-35 Lightning multipurpose fighter, faces reductions, though not cancellation.

He said it would be difficult for all three variants of the fighter jet to survive the current budget scrubbing.

Analysts say the Pentagon may terminate the Marine Corps’ vertical takeoff and landing version of the F-35.

The bipartisan, 12-member supercommittee must reach a deal on reducing the deficit by $1.5 trillion next month, or else $1.2 trillion in spending cuts will be made — most of which will be borne by the Pentagon.

On Thursday, several congressional Democrats sent letters to the supercommittee urging its members to aggressively find savings but also to be cautious about cutting programs that lawmakers care about most, the Associated Press reported.

Republicans and Democrats on the House Armed Services Committee told the supercommittee in separate letters that the Pentagon’s budget already has been reduced sufficiently and additional cuts would hurt national security.

Republicans have argued that most of the government’s savings should come from spending less on government programs, including entitlements such as Medicare, while Democrats have insisted that tax increases be part of a final agreement.