The U.S. military issued formal guidelines today authorizing troops in Iraq to detain civilians who "interfere with mission accomplishment" and hold them for up to 30 days in an effort to combat paramilitary fighters who are disguising themselves as civilians.

Marines patrolling the battleground city of Nasiriyah and other hotly contested areas have already rounded up more than 300 Iraqi men wearing civilian clothes on suspicion of involvement with President Saddam Hussein's Fedayeen or other militias that have been ambushing U.S. supply lines.

While not initially designated as prisoners of war under international law, the detainees will be treated the same way, according to the new rules of engagement, drafted in recent days. A copy of the rules was made available today.

Military commanders have said those who are found to have used civilians as human shields or otherwise violated the laws of war could be deemed "illegal combatants" and shipped to detention centers such as the one at the U.S. naval base at Guantanamo Bay, Cuba.

"Civilian noncombatants are protected persons" under the Geneva Conventions of 1949 that govern conduct during wartime, the new guidelines say. "However, when necessary for imperative reasons of security, such civilians may be temporarily detained." They can also be held if they "possess information important to mission accomplishment."

The guidelines say that "temporary detention should be understood in terms of mere days" and that anyone likely to be held for more than 30 days would have to be processed and transferred to the rear, where higher military authorities would take over the case.

The guidelines instructed soldiers and Marines to handle such detainees humanely. "The detainee must be disarmed, secured, and watched, but he must also be treated at all times like a human being," the guidelines said. "He must not be tortured, killed, or degraded. Likewise, do not engage in conversation with detainees, or offer them any information or comfort items (i.e. cigarettes, candy, etc.)."

The military has been rounding up civilians in an attempt to counter an enemy that U.S. troops say is exploiting innocents in attacks on Americans in southern Iraq. Fighters have been disguising themselves as civilians, faking surrenders, pushing women and children into the line of fire and sniping at U.S. troops from within crowds of civilians, troops say.

As an example of such illegal tactics, U.S. officers site a suicide bombing that killed four Americans on Saturday. A man dressed in civilian clothes drove a car up to a U.S. Army checkpoint near the city of Najaf, waved to soldiers as if seeking help and when they drew near blew up his vehicle.

Field commanders have issued orders to their troops to be more vigilant at such checkpoints and authorized them to open fire if necessary.

At another checkpoint on Monday, U.S. troops blasted an approaching vehicle carrying as many as 16 people, most of them women and children, in the belief that an attack was underway. Ten people in the vehicle died. Soldiers said later that they fired warning shots that were ignored.

According to Human Rights Watch, a private advocacy group, many of the tactics used by Iraqi paramilitary forces violate international law that prohibits attacks based on "perfidy." Deceiving opposing forces into believing that attackers are really noncombatants so that the enemy drops his guard qualifies as an act of perfidy, according to the group. Such actions are dangerous because they blur the line between civilians and combatants and make it more likely that soldiers will fire on innocents.

"When combatants disguise themselves as civilians or surrendering soldiers, that's a serious violation of the laws of war," said Kenneth Roth, executive director of Human Rights Watch. "Any such blurring of the line between combatant and noncombatant puts all Iraqis at greater risk."

The guidelines also provide tips for recognizing possible paramilitary fighters. They are described as men aged 18 to 35, better fed or more physically fit than average and unable to explain their presence in a combat zone. One member of a group might act scared or nervous, because he has been forced to fight. "Marines should be able to sense something is 'off,' " the guidelines advise.

Under the guidelines, suspects can be searched, gagged when necessary and interrogated in the field. They should be segregated by rank, sex, military status, religion, level of cooperation and other criteria, the guidelines said, and generally kept silent so that they "cannot plot to resist or escape." Those who need medical treatment should get it and detainees should be allowed to practice their religion.

Iraqi civilians are forced out of their home in the city of Nasiriyah during a roundup by U.S. Marines from the 2nd Battalion, 8th Regiment.