Worried that bombing raids may reach a point of diminishing returns, military officials in Operation Desert Storm are contemplating the selective use of allied ground forces -- short of an all-out assault -- as a goad to flush Iraqi tanks and troops into the open where U.S. warplanes could attack and destroy them.

Defense Secretary Richard B. Cheney outlined the strategy to reporters traveling with him yesterday to Saudi Arabia: "It may well be that one way to make the air campaign more effective is to add other elements to the campaign," Cheney said.

"You add the {Marine} amphibious element or ground forces in the fashion that forces him {Iraqi President Saddam Hussein} to move out of his prepared positions," he said. "It's moving out of those positions that makes him vulnerable once again to the Air Force."

Cheney and Gen. Colin Powell, chairman of the Joint Chiefs of Staff, are visiting Saudi Arabia this weekend to discuss the course of the Persian Gulf War with U.S. forces commander Gen. H. Norman Schwarzkopf and other Desert Storm commanders before making new recommendations to President Bush.

Central to the visit is the question of whether to continue exclusively the air war that began Jan. 17 or to move into a new stage of the conflict that combines a different kind of air campaign linked to a ground offensive.

Senior officials here and at the Pentagon acknowledge that bombers currently are destroying hundreds of Iraqi tanks and some dug-in troops but say the day is approaching when, without some ground attacks, Desert Storm will degenerate to a drawn-out contest between allies bombing harder and the Iraqis digging deeper.

"If we kill 30, 40 or 50 percent" of Iraq's 5,000 tanks "that's going to disorganize the defense," said one high-ranking Pentagon planner. But dumping thousands of more bombs on Iraqi Republican Guard division positions beyond February would inflict relatively little additional damage in the short term, he contended.

Cheney yesterday lauded the effectiveness of the air campaign, but he also suggested that "there will be a point when we'll want to kick in the next stages of the campaign."

Though allied leaders have long indicated that the campaign to force the Iraqis out of Kuwait would likely involve both air and ground forces, some had held out hope that Iraq could be made to withdraw without the allies having to undertake a long and bloody ground offensive. Advocates of reliance on air power alone have contended that bombing is gradually destroying Iraq's war-making capability and will eventually enable allied ground troops to move into Kuwait with little opposition.

This possibility appeared to gain credence in the early days of the war, when allied warplanes, using "smart" bombs and guided missiles, destroyed strategic targets all over Iraq and Kuwait. Military officials say the raids have obliterated Iraq's air defenses, chemical weapons manufacturing capability and defense industry, and crippled much of its command and control structure, oil refining capacity and war-making infrastructure.

But as air strike attention shifted in the past week to Iraqi troop concentrations, damage assessments became murkier. Air advocates say the bombing of Republican Guard units in southern Iraq and front-line troops in Kuwait is slowly but surely destroying many hundreds of tanks and other equipment and demoralizing enemy soldiers to the point where they will be a less viable fighting force.

Those who have seen a ground war as inevitable, however, say that air raids are having only a limited effect on the enemy troops, dug in behind sand dunes and hardened bunkers. A major offensive by heavy armored units will be needed to dislodge them, they say.

Cheney seemed to take the middle ground, saying "it's not a cut-and-dried, either-or proposition." By selectively applying ground forces, he suggested, the entrenched Iraqi tanks could be forced to abandon their bunkers and come into the open, where allied warplanes could more easily destroy them.

"We've always seen this as a combined arms operation," said one senior Pentagon planner, with the Army and Marine Corps ultimately driving Iraqis out of their bunkers and subjecting them to the fire of planes loaded with cluster bombs and rockets and helicopters armed with guns and missiles. "Using iron bombs to dig out troops in trenches takes a long time," said the planner, who added that one recommendation to Cheney and Powell is that at least a few highly mobile ground units be sent against Iraqi defenses this month to test their strength and hopefully punch holes in their lines.

Military planners also say, however, that ground force could be used in many ways, depending on the size of the units involved and the desired objective.

Suggestions included everything from a fake Marine landing on the Kuwaiti coast to a division-sized, or larger, armored probe deep into central Kuwait. The idea, said one senior military official, is "to do something Saddam can't ignore," thereby provoking Iraqi tanks into leaving their bunkers.

One military source said the 18,000 Marines aboard ships in the Persian Gulf and the Arabian Sea offered a "menu" of possibilities. Some maneuvers are intended to "deceive the enemy" by staging mock landings to make defenders shift their artillery and troops to counter a non-existent threat.

At the other extreme, the source said, the Marines could stage an "Inchon-style" landing behind enemy lines, as was carried out during the Korean War, establishing a beachhead and moving inland before the enemy had a chance to react. In this case, the Marine presence would demand a response from the Iraqis, who would then be vulnerable to air attack. The source noted that the coast of Kuwait has "a lot of shallow water" and "a lot of good beaches" perfect for an invasion in force.

Marines inland already have begun trying to bloody Iraqi noses. A senior military source said Marine artillery Thursday approached the Saudi border and fired several rounds at an Iraqi gun emplacement and received fire in return: "We initiated it," the source said. "It's a matter of deception; we shoot and move."

Thursday night, a Marine reconnaissance team slipped into southern Kuwait, the first such foray of the war, the Associated Press reported yesterday, citing the commander of the battleship USS Wisconsin. The ship shelled the Kuwaiti coast to create a diversion for the Marine unit.

Such small-scale forays would be unlikely to induce Iraqis to show themselves in large numbers, other sources noted. Far more effective, said one military official, would be a raid into Kuwait by helicopter air-assault teams from a light infantry unit like the 101st Airborne Division.

Alternatively, the source said, an entire armored division could circle the Iraqi front-line defenses in a flanking move and roll into western Kuwait or Iraq proper, refusing to leave until the tanks came out to challenge them.

One field commander speculated that the allies would throw their full might at Iraq once a decision was made to shift to a ground war.

"You are going to see entire corps moving across the battlefield," said Col. Leroy R. Goff, a brigade commander in the Army's 3rd Armored Division. "We haven't seen elements of that size moving since the North African campaign in World War II."

The scenario discussed by Goff would include tens of thousands of men and hundreds -- if not thousands -- of tanks and armored vehicles streaking across the desert at once.

This last alternative may have to wait for a few weeks, however, since some armored units slated for critical roles in the ground war still have not received their full complement of tanks and aircraft.

The 3rd Division, the 1st Armored Division, also from Germany, and the 1st Infantry Division (Mechanized), of Fort Riley, Kan., were ordered to Saudi Arabia as key elements in the second-stage deployment announced by Cheney last November.

Together they form the VII Corps, designed to serve as the possible spearhead of any general offensive on the Iraqi western flank. VII Corps soldiers began arriving in early December, but the last of the corps' equipment was offloaded from ships only in the last few days. It will be a few weeks before the corps is ready to fight.

"The Army's arrival will terminate the deployment phase," said one official Thursday in Washington. "The next question is when the ground combat begins."

Cheney and Powell also met today with the exiled emir of Kuwait, Jabir Ahmed Sabah, and agreed that the 7,000 Kuwaiti ground troops fighting with the allied forces would participate in any ground assault on Iraqi positions. Officials said they expect other Arab forces also to take part.

Powell said he promised the emir that allied forces would try to keep damage to his country to a minimum.

According to congressional sources, Bush must wrestle with something of a dilemma as he approaches the ground war decision. If he waits too long to launch the ground offensive, he may risk further inflaming Arabs and others, including the Soviet Union, who have criticized the steady bombing of Iraq. But if he attacks Iraqi defenses too soon in the wrong place, he risks high American casualties, which could undercut U.S. public support for the war.

"It's essentially a political problem," said a Pentagon official. "We will not run out of bombs. We have everything we need to do this forever."

"I hope we fight this one smart and don't rush in there," said one general. "Leave those guys in their holes," he said of the dug-in Iraqi troops. "They can't go anywhere. They've got it worse than we do."

Gugliotta reported from Dhahran, Saudi Arabia. Wilson reported from Washington. Staff writer Barton Gellman, traveling with Cheney and Powell in Saudi Arabia, also contributed to this report.