Allied warplanes have shifted the focus of their relentless bombing blitz in an attempt to isolate Iraqi forces on the battlefield in Kuwait ahead of a ground offensive.

The U.S.-led coalition wants to cut Iraqi supply lines and communications with Baghdad and deprive Iraqi troops of everything from sleep and food to life itself.The biggest guessing game in Saudi Arabia is when the ground war will start, and the betting is sooner rather than later: perhaps Feb. 14, on which there will be a moonless night, but maybe in 10 days, two weeks or even a month.

For President Bush, who wants to keep casualties as low as possible, the decision will likely hinge on allied assessments of bomb damage to Iraqi forces dug in along the border, a subject of some debate.

Earlier this week, Maj. Bob Baltzer said the Air Force hopes to destroy 50 percent of Iraq's forces before a ground offensive with saturation bombing of ground troops, using a mix of B-52s and faster strikes by smaller jets. Privately, other U.S. officers have confirmed the figure.

British Defense Secretary Tom King said Friday the allies would not launch a ground assault unless they were confident of having destroyed 50 percent of the Iraqi capacity. But Marine Brig. Gen. Robert Johnston said the United States had not established any percentage that would suffice to start a ground war.

Air strikes have shifted from strategic targets to Iraqi front-line troops in Kuwait and southern Iraq. King predicted that air attacks will pick up.

Estimates of how much of their military might the Iraqis have lost so far start at 15 percent to 20 percent, by King. At the higher end, Air Force Col. John McBroom estimated Tuesday that some Iraqi units have had their might cut roughly in half.

Johnston said Iraqi supply lines into Kuwait have been cut by up to 90 percent, and the British said half the strategic bridges from Iraq into Kuwait, said by U.S. forces to number about 40, had been put out of action.

The U.S. military command said Saturday that 750 tanks, about 10 percent of Iraq's total of 4,000, and 650 artillery pieces, about 20 percent of its total of 3,200, had been confirmed destroyed.

The difficulty with assessing the effectiveness of the air war, however, is the absence of independent, on-the-ground confirmation of bomb damage.

"That magical figure - that we must reduce Iraqi strength in Kuwait by 50 percent - that's nonsense," said Francis Tusa, European editor of Armed Forces Journal International. "It's irrelevant. You'll never know."

He said the sheer number of bombing missions has to be taking a toll, but it might be on only the least protected targets, not the most entrenched. Last week, pilots said there were still plenty of targets, but prime targets were harder to find and they were trying to destroy Iraqi armor tank by tank.

Tusa said more than 1,000 air sorties daily are combat runs and if each aircraft caused only one casualty, that would mean more than 7,000 troop deaths every week.

Reports that Iraq's best-trained Republican Guards have lost 20 percent to 30 percent of their strength in two weeks, if true, would be impressive, he added.