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 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 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.