WASHINGTON -- Much of Iraq's tactical chemical arsenal is made up of weapons that are fired by artillery or short-range rocket launchers, and generally can only hit targets a few dozen miles away or less, U.S. officials say.
Most of the weapons - including modern nerve agents and World War I-era mustard gas - are held by highly-trained Republican Guard units, who pose the most immediate threat to U.S. forces in the field.
U.S. officials, who spoke on the condition of anonymity, said the units were most likely to fire chemical warheads to cover a retreat or to put down an internal uprising. American troops are going in with gear to protect them from such weapons.
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."
U.N. weapons inspectors have not yet uncovered such arsenals, but U.S. intelligence officials remain convinced Iraqi field forces are armed with the weapons.
U.S. forces will most likely encounter Republican Guard troops as they close in on Iraqi President Saddam Hussein's centers of power. Iraq has six Republican Guard divisions, numbering between 10,000 and 12,000 men each.
Four divisions are arrayed around Baghdad; a fifth is moving a significant part of its force to Saddam's hometown of Tikrit, a city north of Baghdad where much his power is concentrated. A sixth remains near Kirkuk in northern Iraq.
Saddam probably trusts only the Republican Guard with chemical or biological weapons; his regular army is not believed to have any, U.S. officials said.
While estimates suggest he may have 20 or 30 Scud missiles capable of carrying chemical or biological weapons, officials say he 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 more battlefield utility.
Saddam's biological weapons, particularly anthrax, are more apt to strike civilian targets, 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 also risk terrorist-style attacks originating in Iraq.
Most of Saddam's chemical and biological arsenal consists of short-range rockets and artillery warheads, as well as free-fall bombs that would be dropped from aircraft. He also has some exotic sprayer weapons that can be mounted on helicopters, fighters and unmanned airborne drones, U.S. intelligence officials say.
His aircraft probably wouldn't survive long against vast U.S. air superiority, Cordesman said. That leaves the short-range battlefield weapons as the likeliest means of employing the arsenal.
Saddam tightly controls his security forces, but some officials worry he will decentralize the chemical weapons command, ordering units to use them if U.S. bombing cuts off communication with headquarters.
Secretary of State Colin Powell, citing unspecified intelligence sources, told the U.N. Security Council that Saddam has 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."
Saddam is believed to have deployed weapons containing VX and sarin, two extremely deadly nerve agents, U.S. intelligence officials say. He is also believed to have significant quantities of mustard, one of the deadlier World War I chemical weapons.
The Pentagon has e-mailed Iraqi generals to encourage dissent and defections and warn them against using chemical or germ weapons against U.S. or allied forces.
Iraq used chemical weapons on Kurdish insurgents and Iranian forces in the 1980s, killing thousands.
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