The $600 billion in new cuts would kick in only if a bipartisan congressional panel cannot reach agreement on $1.2 trillion in budget savings over the next decade.

But even the prospect of such cuts has prompted officials this week to sound the alarms, with a senior defense official warning that the military could be forced to furlough thousands of employees and Panetta issuing a message to Defense Department personnel declaring that cuts of $600 billion would be “completely unacceptable” to him and to the president.

The Pentagon was already planning to trim spending by about $400 billion -- cuts that Panetta said Thursday were within reason. But he said that the Pentagon should be exempted from additional reductions.

The cuts that could be imposed as part of a so-called “sequestration process,” he said, would have a major impact on national defense.

“If it happened, and God willing that would not be the case, but if it did happen, it would result in a further round of very dangerous cuts across the board – defense cuts that I believe would do real damage to our security, our troops and their families, and our military’s ability to protect the nation.”

Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, echoed Panetta’s concerns, saying: “To loosen the pin unnecessarily through debilitating and capricious cuts ... puts at grave risk not only our ability to accomplish this missions we have assigned, but those we have yet to be assigned as well.”

Defense spending represents about half of the federal government’s discretionary spending, and the Pentagon’s base budget has more than doubled since 2001. While the Defense Department has incurred hundreds of billions of dollars in costs during the conflicts in Iraq and Afghanistan, nearly half of the growth in defense spending over the past decade has been unrelated to the wars.

Panetta and Mullen acknowledged Thursday that the Pentagon needed to rein in costs after more than a decade of growth. But they said it was already doing its part in trimming $350 billion to $400 billion.

“As I have said many times, our growing debt remains the single, biggest threat to our national security.  The military exists to eliminate or mitigate security threats. So, we will do our part in this regard,” Mullen said. “But we cannot allow that effort to go so far and cut so deep that it jeopardizes our ability to deal with the other very real and very serious threats we face around the world.”

Pentagon officials have tried to sound optimistic about the chances of a congressional panel reaching a deal that would spare the Pentagon additional pain. That hasn’t stopped them, however, from citing the dangers should the committee of Democrats and Republicans fail.

Asked whether, given the sharpness of his warning over the cuts, Panetta thought he would still be able to serve as defense secretary, he chuckled.

“I didn’t come into this job to quit,” he said. “I came into this job to fight.”