WASHINGTON -- Iraqi forces guarding Baghdad are armed with chemical weapons and may have orders to use them, U.S. officials say, raising the grim prospect of American troops closing in on the capital city and facing a battlefield filled with deadly agents.

Because these Iraqi units are protecting the approaches to Iraq's largest city, the possibility of chemical weapons being used near populated areas also raises the possibility of unprotected civilians being exposed to such weapons, officials say.

U.S. troops have sensors and protective gear designed to shield them from chemical attack. Their tanks and other armored vehicles can seal them off from a battlefield where chemical weapons are used.

Still, "a couple of lucky hits can produce several hundred or thousands of casualties," said Anthony Cordesman, an Iraq expert with the Center for Strategic and International Studies. "Nobody in the U.S. military can discount that risk."

The Iraqi Republican Guard controls the bulk of Iraq's chemical weaponry, most of which can be fired from artillery guns or short-range rocket launchers, according to U.S. officials, who discussed the intelligence information on the condition they not be identified. These weapons can generally hit targets from a few dozen miles or less.

Saddam Hussein has arrayed most of his Republican Guard units around Baghdad, officials say. Of his six divisions -- each numbering between 10,000 and 12,000 well-trained troops -- they say four defend Baghdad, while a fifth is moving a significant part of its force to Saddam's hometown of Tikrit, a city north of Baghdad where much of his power is concentrated. A sixth remains near Kirkuk in northern Iraq.

The weapons of greatest concern include the nerve agents sarin, cyclosarin and VX, as well as World War I-era mustard gas, officials said.

When used, all but sarin can linger on a battlefield for hours or even days, although experts say these won't spread far. While sarin breaks down quickly, it puts out deadly vapors before it becomes inert.

The Iraqi forces are most likely to fire chemical warheads to cover their retreat or to put down an internal uprising, officials said. Civilian deaths would be all but certain in populated areas, officials said.

But whether Iraq would use these weapons is unknown.

Secretary of State Colin Powell, citing unspecified intelligence sources, told the U.N. Security Council that Saddam had authorized some of his field commanders to use chemical weapons.

In recent testimony to Congress, CIA director George J. Tenet said, "Do I know whether his subordinates will take the orders? I don't know. There are some unknowables, but you must plan as if he will use these weapons."

The Pentagon has tried to e-mail Iraqi generals to warn them against using chemical or germ weapons against U.S. or allied forces.

U.N. weapons inspectors have found only a few pieces of Saddam's allegedly vast arsenal, but U.S. intelligence officials remain convinced Iraqi field forces are armed with the weapons.

While estimates suggest Iraq may have 20 or 30 Scud missiles capable of carrying chemical or biological weapons, officials say Saddam's military may have 30,000 artillery warheads capable of carrying chemical weapons and 550 artillery shells filled with mustard agent.

Chemical weapons can kill quickly and therefore have battlefield utility, while Saddam's biological weapons, particularly anthrax, are more apt to strike civilian targets because they can take hours or days to take effect, U.S. officials said.

Much of the concern about Saddam's arsenal has been directed at its potential use against political targets -- particularly Israel, Saudi Arabia, or Kuwait -- using Scud missiles. Some have suggested these and U.S. targets are also at risk of terrorist-style attacks originating in Iraq.

While Saddam also has planes equipped with bombs and spray tanks, experts predicted these would not survive long against superior U.S. fighter aircraft.

That leaves the short-range battlefield weapons as the likeliest means of employing a chemical arsenal against U.S. forces. Experts said these weapons function by detonating above the ground, spraying the deadly chemicals as both liquid droplets and aerosol over a wide area.

Many shells or rockets are required to contaminate even a few dozen acres of land.

Iraq used chemical weapons on Kurdish insurgents and Iranian forces in the 1980s, killing thousands.