Pentagon officials said they might furlough employees if lawmakers failed to cancel spending reductions. The Post’s Felicia Sonmez explains how going over the “fiscal cliff” will affect you and your wallet. (The Washington Post)

With lawmakers unable to approve a deal that would have averted steep spending cuts, Pentagon officials said Monday that 800,000 civilian employees could be ordered to go on unpaid leave for periods of time.

The military’s service chiefs, who have already been making cuts as part of a separate, long-term effort to whittle down the defense budget, are working to assess the impact of the congressionally mandated cuts. The broad fiscal retrenchment would begin Wednesday, although it is possible that lawmakers will find ways in coming weeks to allocate separate funding for the Pentagon and avoid the furloughs.

“Senior leaders in the department are working hard on how to communicate to the workforce what the consequences might be,” a senior defense official said Monday, speaking on the condition of anonymity to discuss the Pentagon’s contingency planning.

Neither party wants to see a dramatic drop in defense spending.

During a recent trip to Kuwait, Afghanistan and Turkey, Defense Secretary Leon E. Panetta told U.S. service members that he was deeply worried about the effect the cuts could have on national security and military readiness. The Pentagon would have to slash $500 billion over the next decade on top of cuts it has been working toward as the country eases off a war footing, Panetta said.

“That would do significant harm to our ability to provide strong defense for the country,” Panetta told U.S. troops in Kuwait. “It would be reckless for the Congress to allow that to happen.”

If the Pentagon is forced to proceed with the reductions, part of a process known as sequestration, officials might be forced to make significant personnel cuts to the Army and Marine Corps, which are already in the midst of a gradual force reduction. The department would also be likely to have to put on hold or downscale costly weapons system upgrades and purchases.

In addition, the military would be likely to impose a hiring freeze and review its contracts, in some cases declining options to extend them, according to Army Lt. Col. Elizabeth Robbins, a Pentagon spokeswoman.

Panetta and other senior defense leaders called key lawmakers over the past two days warning that “over 800,000 civilian employees could face furloughs relatively soon should sequestration occur,” the defense official said. “That’s not something that anyone wants to see happen.”

Panetta told Pentagon employees in a memo issued Dec. 20 that military personnel rosters would not be affected by sequestration during the 2013 fiscal year, noting that the White House has said it would use its authority to protect the force in the short term. Civilian employees would be vulnerable to furloughs, a type of unpaid leave, however.

“Let me assure you that we will carefully examine other options to reduce costs within the agency before taking such action,” he wrote.

Michael E. O’Hanlon, a defense-spending expert at the Brookings Institution, said that after the deadline expires, incoming lawmakers could hammer out an agreement over the next two months to allocate funding to federal agencies.

If sequestration occurs, he said, the Obama administration would have to either downscale its mid- and long-term defense strategic planning or take a fresh look at what it can realistically accomplish with substantially less money.

“There are some things that are risky but not necessarily incompatible with the thrust of our foreign policy,” O’Hanlon said Monday, citing as examples modest trims in forces and slight increases in health-care premiums.

A more dramatic approach, he said, could entail cutting the Army by 25 percent, which would leave enough ground troops for a major war but not two concurrent conflicts, or slashing pay for uniformed personnel by thousands of dollars.

Lisa Rein contributed to this report.