They are ill-trained, lightly armed and highly motivated.
Thousands strong, Saddam Hussein's personal hard-core paramilitary groups have become the wild card in the war against Iraq.
U.S. military planners who believed their biggest test in Iraq would be squaring off against Hussein's Republican Guard units now find themselves slowed and frustrated by a stubborn, elusive guerrilla campaign they had hardly reckoned as a significant force.
Since the invasion began, paramilitary fighters have organized deadly ambushes against U.S. supply columns on the road north toward Baghdad. Sunday, for example, they killed members of an Army maintenance unit near Nasiriyah and captured others. Paramilitary commanders also are believed to be leading resistance in the city.
Paramilitary forces have held the country's second-largest city, Basra, tightly in their grip and forced reluctant Iraqi soldiers in the city to rejoin a fight they had given up. They also have slowed the reopening of Iraq's main port town, Umm al Qasr, and have organized resistance in other towns.
Now there is evidence that they may be gathering young civilians stirred by Iraq's initial resistance and launching them on suicide charges directly into allied fire.
In some cases, these attacks are both futile and bloody, with young men, armed only with rakes and pitchforks, running barefoot at U.S. troops, said Col. Ben Saylor, 1st Marine Division chief of staff.
The fighters, generally between 18 and 35 years old, tend to hide behind agricultural berms -- mounds of dirt that separate fields and often are used as irrigation channels. They then jump up and charge at Marine positions from as far as 100 yards away, said Saylor.
"They will charge as a human wave, a dozen of them coming right at a position, and they'll get mowed down," Saylor said. "Then another 12 will jump up and charge and they'll be killed."
Some charges are coordinated, albeit ineffective -- part of delayed ambushes that begin with a rocket-propelled grenade or small arms fire on the U.S.-led forces from one side of the road, followed by a mass charge from the other side by men with small hand weapons.
"They're showing a lot of guts, but without the tactics and weapons of modern warfare it's not going to work," said Marine Capt. Dave Nettles, an assistant intelligence officer with the 1st Marine Division. "You feel sorry for them. You don't want to start killing civilians, but when a guy brandishes a weapon and charges at you, what can you do?"
Marine operations officer Clarke Lethin said the fighters harassing the 1st Marine Division in central Iraq appeared to be from the Al Quds Army -- a group also known as the Jerusalem Army that formed about three years ago in Iraq with the goal of seizing the West Bank and turning it into an independent Palestine.
"Some are volunteers who recently joined the group for the prestige of it," Lethin said. He compared them to Nazi brownshirts: "not well-trained but a fanatical bunch."
U.S. military intelligence officers believe that others have been coerced into joining through threats against family members.
"There could be elements of both, but I would have little doubt that some of them are there under duress," said Jonathan Stevenson, a counter-terrorism specialist at the International Institute for Strategic Studies in London.
In Basra, British military sources believe the presence of several hundred members of the Fedayeen Saddam, a paramilitary arm of Iraq's internal security network loyal to Hussein's elder son, Uday, have kept the city from falling to the allies. They also believe the group is behind the bizarre, seemingly aimless dash of Iraqi armored columns that have broken out of the city in fighting formation on each of the last three days.
As on the previous two occasions, allied aircraft and British tanks destroyed the entire column.
Speaking to reporters Thursday, the commander of British forces in Iraq, Air Marshal Brian Burridge, said he believed the armored columns had been part of the Iraqi Army's 51 Mechanized Division, which disintegrated in the initial days of the war, when many soldiers took their vehicles into Basra. Now, he said, paramilitary groups were forcing the soldiers to fight.
"They rule above the law, intimidating regular army soldiers, by executing some soldiers or family members of soldiers who reside in the city," Burridge said.
Despite the early successes of the Al Quds Army and other groups in disrupting the advance of U.S. forces, they seem unlikely to shift the course of the war, Saylor and other divisional leaders said.
"We've been contested every inch of every mile," Saylor said. "Does it annoy us? Yes. Is it going to stop us? Not even on a bad day."
Some analysts argue that once Hussein's power is broken, these groups will simply disappear as leaders flee or go underground to avoid capture.
A deeper worry, however, is that they will go underground once the war ends and mount a new campaign of terrorism against a U.S.-administered postwar Iraq.
"The risk that's materializing is that even after the battle is won, there will be a residual terrorist problem," counter-terrorism expert Stevenson said.