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Defense Secretary Leon Panetta warned lawmakers Thursday not to enact a “doomsday mechanism” that would slash an additional $600 billion from the Pentagon budget, saying that such cuts would pose a severe threat to U.S. national security.

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.”