Hundreds of men trying to flee the assault on Fallujah have been turned back by U.S. troops following orders to allow only women, children and the elderly to leave.

The military says it has received reports warning that insurgents will drop their weapons and mingle with refugees to avoid being killed or captured by advancing American troops.

As it believes many of Fallujah's men are guerrilla fighters, it has instructed U.S. troops to turn back all males aged 15 to 55.

"We assume they'll go home and just wait out the storm or find a place that's safe," one 1st Cavalry Division officer, who declined to be named, said Thursday.

Army Col. Michael Formica, who leads forces isolating Fallujah, admits the rule sounds "callous." But he insists it's is key to the mission's success.

"Tell them 'Stay in your houses, stay away from windows and stay off the roof and you'll live through Fallujah,'" Formica, of the 1st Cavalry Division's (search) 2nd Brigade, told his battalion commanders in a radio conference call Wednesday night.

Many of Fallujah's 200,000 to 300,000 residents fled the city before the assault, at which time 1,200 to 3,000 fighters were believed in militant stronghold.

Later Prime Minister Ayad Allawi (search) imposed a 24-hour curfew on Fallujah and ordered roads in the area closed, providing the legal background for the U.S. blockade.

Troops have cut off all roads and bridges leading out of the city. Relatively few residents have sought to get through, but officers here say they fear a larger exodus.

On Wednesday, a crowd of 225 people surged south out of Fallujah toward the blocking positions of the Marines' 2nd Reconnaissance Battalion. The Marines let 25 women and children pass but separated the 200 military-age men and forced them to walk back into Fallujah.

"There is nothing that distinguishes an insurgent from a civilian," the 1st Cavalry officer said. "If they're not carrying a weapon, you can't tell who's who."

Also Wednesday, troops halted two ambulances leaving Fallujah and found 57 refugees packed inside. Most were women and children who were allowed to leave. Smaller bands of refugees have also turned up at U.S. roadblocks, some allowed to pass and others turned back.

Single refugees have made their way out of the city by swimming across the broad Euphrates River or sneaking out across desert paths, military officials said.

On Wednesday and Thursday, American troops sunk boats being used to ferry people — and in some cases, rebel arms — across the river.

The ongoing U.S. advance is bottling up Fallujah's insurgents — and others fleeing the fighting — in the southern section of the city, where U.S. forces were moving Thursday night.

Most of the remaining attacks by insurgents inside Fallujah have been on Marines blocking the roads and bridges leaving the city, reports show. Marines have returned fire killing numerous insurgents trying to escape, officers here said.

The military estimates 600 insurgents have already been killed, about half the total of guerrillas in the city.

Fallujah has been under relentless aerial and artillery bombardment and without electricity since Monday. Reports have said residents are running low on food. An officer here said it was likely that those who stay in their homes would live through the assault, but agreed the city was a risky and frightening place to live.

U.S. military says it does all it can to prevent bombing buildings with civilians inside them.

Once the battle ends, military officials say all surviving military-age men can expect to be tested for explosive residue, catalogued, checked against insurgent databases and interrogated about ties with the guerrillas. U.S. and Iraqi troops are in the midst of searching homes, and plan to check every house in the city for weapons.