A war with Iraq could cost the United States as much as the Vietnam War did in its most expensive year, according to government estimates and private defense experts.

The government estimates, including a Congressional Budget Office report released yesterday, say that a war to expel Iraqi forces from Kuwait could add anywhere from $28 billion to $86 billion, spread over two years or more, to the non-combat costs of Operation Desert Shield.

The "enormous range" of the projections, said Sen. Jim Sasser (D-Tenn.), who released the CBO report, suggests "the unpredictability of what we're getting ourselves into if we get into a war in the Middle East." Sasser cancelled a hearing on war costs last week when the witnesses from the State and Defense departments backed out of earlier plans to testify.

Cost projections, like casualty projections, reflect deep uncertainty among analysts about how long war with Iraq would last and how heavy American losses would be.

The lower CBO estimate of $28 billion is based on war scenarios developed by Joshua Epstein of the Brookings Institution and retired Col. Trevor N. Dupuy, a military historian. It assumes that if fighting erupts in the Persian Gulf, it would last less than a month and result in 3,000 U.S. casualties and the loss of 200 tanks and 100 aircraft.

The higher estimate of $86 billion is based on a longer and more costly scenario by the Center for Defense Information, which assumes one to six months of combat with 45,000 casualties and the loss of 900 tanks and 600 aircraft.

Another major uncertainty, according to independent analysts, is one not raised in the CBO analysis: Would the Pentagon seek to replace all or even most of the equipment it loses in a gulf war, given the cutbacks in defense spending that were planned before Iraq invaded Kuwait?

Destruction of such high-technology equipment as front-line fighter aircraft, main battle tanks and helicopters represents the largest category of costs in the CBO report, which assumes that all such losses would be replaced. The average fighter plane in Air Force and Navy arsenals was bought for about $25 million. Each air-to-air missile used by fighters against other planes in aerial combat cost between $100,000 and $500,000. The average combat tank deployed in the gulf was purchased for about $3 million.

But because the Pentagon is already slated to cut its stocks of weapons and equipment, budget planners might choose to regard wartime losses as nothing more than a violent and unforeseen method of making already planned reductions, according to analyst Barry R. Posen of the Massachusetts Institute of Technology.

"It's a kind of macabre way of looking at it, but . . . we were going to retire it or get rid of it or break it up in arms control, and the actual replacement of this equipment is not necessary," Posen said.

Other analysts noted, however, that the Pentagon never intended to retire or destroy its best high-technology equipment, such as the most modern versions of the F-14, F-15 and F-16 aircraft or the F-117A "stealth" fighter.

Other major cost categories, all of them dependent on the duration and intensity of combat, include fuel, food, spare parts, maintaining troops and equipment, replacing spent ammunition and caring for wounded troops after the war.

The Vietnam War, in its peak year of 1969, cost the equivalent of $85 billion in 1991 dollars, according to Gordon Adams, director of the Defense Budget Project, a private think tank. That amounted to about $233 million a day, in today's dollars. Throughout that war, Pentagon budgets ran far beyond projections.

Adams said that because a gulf war would be particularly intense and use more expensive high-technology equipment, a more apt cost comparison should be the three-week 1973 Yom Kippur war, which the Israeli ministry of finance has suggested cost the equivalent of about $750 million a day in 1991 dollars.

"That is the closest to a comparable standard," Adams said. "A brief war with lots of equipment loss."

The Pentagon has not released its own estimates of how much it might cost to fight Iraq.

Staff writer John E. Yang contributed to this report.