WASHINGTON -- Tens of thousands of paramilitary and guerrilla fighters loyal to Saddam Hussein pose one of the biggest obstacles to American efforts to quickly topple Iraq's government.
Defense Secretary Donald Rumsfeld has called them "dead enders," Iraqis who would fight to the end. U.S. officials, worried they may have underestimated these fighters, have begun calling them terrorists and war criminals and focusing significant military efforts on wiping them out.
"This band of war criminals has been put on notice," President Bush said Wednesday of fighters he said were using civilians as human shields. "The day of Iraq's liberation will also be a day of justice."
Pentagon officials had expected at least some hard-core Saddam loyalists to resist, but had assumed they would make their stands mostly during the climactic battle for Baghdad. Privately, they acknowledge being startled at the irregulars' tenacity and brutality.
Now the fighters -- tens of thousands of lightly armed but highly mobile paramilitary troops and members of Saddam's internal security police -- are causing problems for the United States before the battle for Baghdad.
By sporadically attacking long U.S. supply lines in southern Iraq, groups like the Fedayeen have forced the United States to pause and redirect some troops to fighting back.
By holing up and resisting in southern cities like Basra, they also are preventing regular Iraqi soldiers from surrendering and delaying crucial humanitarian aid to ordinary Iraqis.
Their impact might be relatively short-lived because, unlike in Afghanistan, these hard-core fighters have no caves to hide in and few ways or motivation to escape the country. But one place where they can fight -- Iraq's cities -- could make digging them out difficult and bloody.
If they can successfully hole up and fight in several cities at once, that "could confront the U.S. with much more serious problems in urban warfare," said Anthony Cordesman, a military analyst at the Center for Strategic and International Studies in Washington.
U.S. officials insist the irregulars are not hurting the overall war effort, noting for example that U.S. supply lines continue to move.
But Maj. Gen. Stanley McChrystal said the guerrilla fighters also have a propaganda aim. "They're trying to get an over-reaction from coalition forces, so that we'll fire on people who are trying to surrender."
Their numbers in southern Iraq are believed in the low tens of thousands, mostly Fedayeen, plus some militia from his ruling Baath Party and others, say U.S. intelligence officials. They are lightly armed but highly mobile, using civilian vehicles to get around. They generally operate in small groups.
McChrystal said some special Republican Guard units -- Saddam's best paramilitary group -- also had been sent down from Baghdad to "stiffen" regular army forces.
U.S. intelligence has detected hundreds of suspected paramilitaries moving south from Baghdad in civilian vehicles like pickup trucks and sport-utility vehicles in recent days, said one defense official.
U.S. officials insist the fighters' tactics sometimes amount to war crimes.
Brig. Gen. Vincent Brooks said Iraqi fighters both in uniform and out had marched civilians in front of formations as they fired and also had pretended to surrender before opening fire.
At least one report had Iraqi irregulars wearing U.S. uniforms, accepting the surrender of Iraqi troops and then executing them, said Pentagon spokeswoman Victoria Clarke.
"I'm not going to call them troops because they're ... essentially terrorists," Rumsfeld said Tuesday.
Their reported weapons so far have ranged from AK-47 rifles to rocket-propelled grenades and even tanks.
They are thought to include some:
-- Special Republican Guards, a 12,000 to 25,000-man paramilitary force that protects Saddam's presidential residences. They are believed the most well-trained Iraqis for urban warfare.
-- Saddam Fedayeen, a 20,000 to 25,000-man militia created in 1995 from tribal areas loyal to Saddam to combat internal unrest, especially among Shiites. A fierce fight Tuesday pitted the U.S. 7th Cavalry of the 3rd Infantry Division against a force of both regular Republican Guard units and Fedayeen fighters, officials say.
-- Special Security Service Organization and other internal security police reporting directly to Saddam's inner circle. These are not military troops, but enforcers used to guarantee that regular army troops and even the Republican Guard keep fighting.