A senior Pentagon official warned Wednesday that the military would be forced to furlough or lay off thousands of employees if it is required to cut an additional $600 billion from the defense budget.

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

The panel’s failure would trigger cuts of about equal amounts from defense spending as from entitlement programs over the next 10 years.

Senior Pentagon officials walked a fine line Wednesday in raising alarms about the possibility of large cuts to the defense budget and at the same time casting the big cuts and possible layoffs as virtually unthinkable.

“I would rather avoid scaring our employees,” said the senior defense official who spoke on the condition of anonymity to Pentagon reporters. “I don’t think this will happen.”

The Pentagon briefing came on the same day that Defense Secretary Leon E. Panetta sent a letter to service members and other Pentagon employees in which he said that major cuts to the defense budget would be “unacceptable.”

“This potential deep cut in defense spending is not meant as policy,” he wrote. “Rather it is designed to be unpalatable to spur responsible, balanced deficit reduction and avoid misguided cuts to our security.”

The big cuts to the defense budget would not affect funding for the war in Afghanistan, but they would require the department to shed civilian workers and possibly lead to the involuntary separation of some service members, the senior defense official said.

The Pentagon already has agreed to trim about $350-$400 billion from its current spending plans over the next decade. Those savings would essentially limit the growth in the Pentagon budget to about the rate of inflation.

“We think that is hard to do, but manageable,” the official said. “Now our job is to figure out how to do it.”

The Pentagon is expected to spend about $553 billion during the 2012 budget year, a figure that doesn’t include much of the war spending for Afghanistan and Iraq.

Many defense analysts have speculated that the Pentagon would be compelled by the bipartisan panel to make further cuts beyond the $350 billion to $400 billion planned over the next decade. But the senior Pentagon official said that the department was optimistic that the panel of Republicans and Democrats would reach an agreement that largely spares the Pentagon any additional pain.

“I would expect their focus to be on entitlements and taxes,” the senior defense official said. “I would hope they would not make further cuts from defense.”

Panetta is expected to hold his first Pentagon news conference Thursday with Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, and will probably address the Pentagon’s expectations regarding the defense budget.

In his letter to Defense Department personnel, Panetta made clear that he would not support big cuts to the budget. “I promised in my first message as secretary that I will fight for you,” he wrote. “That means I will fight for you and your families as we face these budget challenges.”

So far, the Pentagon has not begun preparing for cuts beyond the initial round totaling $350-$400 billion that it was told to make by the White House several months ago, the senior defense official said.