A portion of Iraq's arsenal of chemical weapons is believed to have survived weeks of aerial bombardment by U.S. and allied warplanes and remains ready for potential use against allied ground forces if they speed northward to engage Iraqi troops, according to U.S. intelligence and military officials.

While a variety of battlefield defenses against chemical attack have been prepared by the U.S. Army, U.S. policy-makers and military leaders remain in a quandary about how best to respond politically or strategically if Iraq should employ chemical weapons, the officials said.

U.S. soldiers in northern Saudi Arabia have been trained to conduct offensive operations for several hours clothed head-to-toe in awkward, charcoal-lined protective suits. They have also been equipped with sensors and alarms that would signal the beginning of a chemical attack.

A tentative list of strategic options, including a "visible" acceleration of conventional warfare or a decision to press for post-war trials of Iraqi commanders implicated in such an attack, also has been prepared for forwarding to the White House if the chemical arsenal is unleashed, officials said.

But no decisions have been made, and many officials harbor doubts that the United States will be able to fulfill its public pledge to "respond in the severest possible way" without lowering itself to Iraq's moral level or provoking a backlash in Arab nations by harming innocent civilians.

"There are no good options," said a knowledgeable Pentagon official. He and others confirmed that a U.S. response involving even limited use of chemical or nuclear arms -- effectively ruled out before the war began -- remains out of the question due largely to lasting political problems that such a response would cause.

Officials said more likely -- but still awkward -- responses could include a more concerted and overt effort to kill Iraqi President Saddam Hussein and his top advisers, or a more general acceleration of the air war. "We could load up the whole {U.S.} bomber fleet, and turn it loose," one official said, "but it would then be a weapon of mass destruction," not unlike poison gas.

No estimate is available of the quantity of deadly chemicals believed to persist in the Iraqi arsenal, because of U.S. intelligence uncertainties about their location and difficulties in assessing damage from the hundreds of strikes directed against them, officials say.

A month before the war began, CIA Director William H. Webster and other officials said Iraq was believed to have 1,000 tons of poisonous chemical agents, much of it loaded in bombs, artillery shells, rockets and two types of short-range missiles -- Frog and Scud-B.

Speculation that Iraq also would fit chemical warheads atop longer-range Al-Hussein and Al-Abbas missiles have not been borne out by the 67 firings so far of these missiles on civilian and military targets in Israel and Saudi Arabia. This fact has caused some officials to conclude that Iraq still lacks the capability of placing chemical warheads on the longer-range Scuds.

Secretary of Defense Richard B. Cheney said last week that Iraq's chemical production and storage sites have "for the most part been destroyed," but added that the nation "still has significant chemical capability . . . in the form of weapons that have been deployed with his forward forces."

Other officials have explained that pilots often have no way of knowing whether the special ammunition bunkers they destroy are stocked with chemical arms or conventional munitions. French officials have reported only occasionally finding diluted poison gas residues in air sampling equipment aboard some of their planes flying over targets in Kuwait, indicating sporadic allied success in destroying chemical storage facilities.

Lt. Gen. Thomas Kelly, operations director for the Joint Chiefs of Staff, said last Thursday that some of Iraq's chemical weapons may have lost their potency over the past four weeks due to instabilities endemic in Iraqi production techniques. He said the remaining threat of chemical warfare was "nowhere near as significant as it was at the outset."

But intelligence and military officials note that Iraq retains as many as 450 planes and more than 1,500 artillery pieces and rocket launchers capable of carrying poisonous chemicals. Some of the rocket launchers could easily fire up to 40-to-60 chemical rounds apiece before being targeted in a battle, the officials said.

The Iraqi goal presumably would be to deliver 15,000 or more such rounds on an allied troop concentration, the officials said -- an amount sufficient to fell a battalion-sized formation or slow its movement by forcing troops to don bulky, protective garb.

"When you're on the defensive and facing a large superior strength force, chemical weapons, theoretically, can give you an advantage," and the Iraqis know it, said a senior Army expert on poison gas defenses. Communications can be impeded and operation of high-tech weapons becomes complicated, other officials said.

"There's a tremendous psychological effect," the Army official said, as soldiers struggle to avoid becoming paralyzed by fear. U.S. analysts believe some Iraqi chemicals could persist from a few hours to more than a day in the desert, forcing allied troops to move around -- not through -- any contaminated zone.

The first sign that Iraq had used deadly nerve gas or liquid mustard, a blistering agent, might come from the insistent whooping of a radio-sized metal box at a desert encampment, the beeping of a small alarm pinned to a soldier's chest-strap, or from the appearance of red dots on a soldier's armband woven with chemically-sensitive material.

It also might come from a reddening of skin, a dimming of vision, a difficulty in breathing, or an involuntary twitching of muscles. Upon exposure to nerve gas, they could have but seconds to grab an automatic syringe and jam it against their thigh, ejecting an inch-long needle and a dose of life-saving chemical antidote; mustard victims would swab their skin with treated cloth.

Secretary of State James A. Baker III has been at the forefront of the administration's campaign to deter Saddam from using chemical arms by threatening "the severest possible" political and military consequences. But Vice President Quayle, White House Chief of Staff John H. Sununu, French President Francois Mitterrand, and U.S. military officials have made clear in recent weeks that U.S. nuclear and chemical weapons will not be used in such a response.

Staff writer Rick Atkinson and researcher Ralph Gaillard Jr. contributed to this report.