The Joint Chiefs of Staff are surveying the U.S. defense industry to determine how it could increase production of ammunition, spare parts and other crucial items needed to keep American troops in the fight if war breaks out in the Persian Gulf, Pentagon officials said yesterday.

This action is one of several new signs of the strain President Bush has put on the armed services by deciding to have a fighting force of 400,000 soldiers, sailors, airmen and Marines in the region by January.

The other signs include the call-up of thousands of reservists to support front-line troops in the gulf, the Army decision this week to freeze almost everyone in uniform into their present jobs in case they are needed as replacements and an internal briefing paper, en route to Defense Secretary Richard B. Cheney, warning that the Air Force is on its way to becoming a "hollow force" because of a looming spare parts shortage.

A high-ranking planner involved with the industry survey said the military already has tapped into its NATO stockpile in Europe for "smart" weapons, such as laser-guided bombs, and will continue drawing down such sophisticated armament while the Joint Chiefs' survey of war industry capabilities proceeds. He said the objective is to have sufficient ammunition and other supplies for 30 days of combat.

Smart weapons enable aircraft pilots to release bombs out of the reach of shoulder-fired antiaircraft missiles, such as the Soviet-made SA-7 heat-seeking missile that the Iraqi army has in abundance, officials said. The prospect of Iraqi President Saddam Hussein marching captured U.S. pilots through the streets of Baghdad has given impetus to obtaining sufficient "standoff" munitions, officials said.

At this time, the Joint Chiefs do not foresee the need to reopen closed ammunition plants, one top planner said, but they may request some manufacturers of crucial items to increase output by putting on extra shifts of workers, according to another Pentagon planner. "We're interested in surge capability," he said.

Military planners charged with thinking through "worst-case" scenarios said they are worried about having enough spare parts to keep in fighting condition aircraft not already in the gulf. They said the Air Force stripped many fighter planes based in the United States of spare parts to make combat-ready those sent to the gulf. This means, they said, that many of the aircraft left behind cannot be called to war as replacements for want of spare parts.

The briefing paper obtained by The Washington Post, entitled "Air Force Logistics Health," includes a chart showing that mechanics at Langley Air Force Base, Va., had to take 200 parts off F-15 fighter planes left at home to enable two of the three squadrons of the 1st Tactical Fighter Wing to fly to the gulf.

This left eight aircraft at Langley unable to fly at the time of deployment in August, the briefing paper states. Five of those eight cannibalized F-15s since have been made flyable, according to the paper. Squadrons of RF-4G, F-16 and A-10 aircraft were similarly stripped to meet the emergency, the Air Force said.

One Air Force general said it made military sense to resort to cannibalization to meet the emergency needs of the Operation Desert Shield gulf deployment. But the "Air Force Logistics Health" report stresses that the gulf experience was symbolic of the long range as well as short-range problems of keeping its combat aircraft ready to fight and calls for a fresh look at funding for spare parts.

One Pentagon planner said the dust in Saudi Arabia has caused unanticipated problems in keeping planes and helicopters in flying condition. He said the dust is like talcum powder, not beach sand, and fouls not only engines but sophisticated vision devices for firing modern weapons at night. The powdery sand rises to an altitude of 6,000 feet much of time, disorienting pilots and sometimes causing them to crash into the desert, according to Army aviation leaders who have been analyzing the recent rash of accidents in the gulf.

An analyst at the General Accounting Office said the dust has been so harmful to the Army's tank-killing helicopter, the Apache, that he doubts whether its high-tech night firing devices will work in combat. One GAO study estimates that it is taking 24 hours of maintenance to keep the Apache in the air for just one hour in the gulf region.

The Air Force fears about becoming a "hollow" force are shared by Army leaders. They said this has driven them to impose an Army-wide ban on departures from their service until it is clear who will be needed for gulf duty if a war starts. The two prime needs, they said, are for technical personnel to keep high-tech weaponry in the fight and for highly trained combat leaders.

"Once the shooting starts," one Army manpower expert said, "you always have plenty of generals and colonels. What you need are those corporals and sergeants who know how to fight. If you lose too many of them, the whole unit loses its combat effectiveness."

The high loss rates in the first days of the last desert war fought with modern weapons, the Yom Kippur War between Egypt and Israel in 1973, have generated concern among Pentagon planners about having enough replacements with the right skills in the event of an offensive against Iraqi forces.

One thing the Yom Kippur War demonstrated, planners said, is that soldiers inside a tank are more vulnerable than the tank to armor-piercing munitions. It is important, they said, to have backup crews so a tank that is hit, but not disabled, can be kept in the fight.

"We want to make sure we keep in the people we may need for reinforcement in the gulf," said one Army leader in explaining the freeze on voluntary retirements from the service. Trained tank crews in the United States, for example, could be flown to the gulf in a war to replace crews who are killed but whose tanks were still operable.

With about 40 percent of the Army's 720,000 active-duty men and women deployed or going to the gulf, the Army leader said, a limited number of active-duty personnel are available with the technical and battlefield skills most necessary if war starts. The Army wants to make sure these skilled uniformed men and women are not released even if their enlistments are up or they request retirement.

Gen. H. Norman Schwarzkopf, the Central Command commander most involved in determining what kinds of forces that are sent to the gulf, so far has been reluctant to put reserve units in front-line positions. This means most of the front-line troops must come from active-duty units.

"Schwarzkopf once commanded the 24th Division," noted one Pentagon executive in discussing the commander's skeptical view of reserves. "He knows what the roundout brigade {reservists who train with a specific active-duty division in the expectation of fighting together} can and cannot do." The 24th Infantry Division (Mechanized) at Fort Stewart, Ga., was among the first outfits Schwarzkopf requested for gulf duty, but he let its roundout brigade of reservists stay home. It since has been ordered to active duty, but must undergo a training period before possibly deploying.

These factors plus the lack of a draft have created the manpower crunch confronting the military.

Pentagon planners note that the last time the United States deployed 400,000 people to a troublespot, the active-duty military was much larger. That was 1970 when President Richard M. Nixon had 414,900 active-duty personnel deployed in Vietnam, including 298,600 Army, 25,700 Navy, 50,500 Air Force, 39,900 Marine Corps and 200 Coast Guard personnel.

This comparison shows the contrast between the active-duty force today and the larger force in 1970: Army, 1,322,000 in 1970 vs. 702,170 in fiscal 1991. Navy, 692,000 in 1970 vs. 570,500 now. Air Force, 791,000 vs. 510,000. Marine Corps, 260,000 vs. 193,735.

The narrowed manpower base also has made it more difficult to rotate units in and out of the gulf, as the Pentagon originally had planned. Cheney said the rotation plan has been shelved but not necessarily scrapped.

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, said the Pentagon's reluctance to activate reserve units for front-line duty will be one of the prime issues during hearings his panel will hold on Desert Shield.

"You can't have it both ways," he said. "You can't keep putting money in them {the reserves} if they're no good. If they're no good, we should get rid of them," he said of reserve units designed for combat duty but not activated to ease the manpower crunch as Bush orders more than 400,000 military personnel to be in the gulf by January.