The sign was ominous, the humor dark. Iraqis who live in the neighborhood had suddenly vanished, often an indication that an attack is imminent.

"No way is anything going to happen," joked Staff Sgt. Angel Perez, a Humvee commander, as he watched an Iraqi police convoy drive near the outpost of the platoon from the 3rd Armored Cavalry Regiment. "That convoy is definitely not going to get hit," he said.

Seconds later, a roadside bomb detonated just ahead of the convoy. Nobody was hurt, but the explosion shook the walls of the barracks and sent the men running for weapons and roaring out of the gate in Humvees.

Although security in Iraq has improved, it remains fragile, Lt. Gen. Lloyd Austin, the No. 2 U.S. commander in Iraq, told reporters in the capital Monday.

And nowhere is the fragility more apparent than here in Mosul, 225 miles northwest of Baghdad. Sunni Arab insurgents, routed or weakened in Baghdad and other urban centers since last year, are making a stand in this former bastion of support for Saddam Hussein.

"Since the start of Ramadan, it's been crazy. We've been mortared, we've had fire come at the guard towers, we've had IEDs," said Spc. Erich Hellwig. The 20 soldiers in Hellwig's unit occupy Outpost Rabiy, a converted salvage yard in a western neighborhood shattered by fighting.

"It's getting to the point where people are leaving their boots on when they sleep. You get worried that someone would come in and take out our gate," said Hellwig, who survived an ambush of his convoy last week. In that attack, a rocket-propelled grenade exploded near a U.S. military vehicle, kicking up a cloud of dust but inflicting no casualties.

Hellwig's company recorded 15 attacks, including car bombs, roadside bombs and rocket-propelled grenades, in the week before Ramadan started on Sept. 1 in the large area of western Mosul that it patrols. Attacks in the first six days of Ramadan nearly tripled to 42, the unit's figures show. The unit did not suffer any casualties during that time.

After the attack on the Iraqi police convoy, Perez and his comrades searched through ruined buildings for the bomb's triggerman. They found only civilians who claimed they did not see or hear anything unusual.

As the American vehicles departed, an insurgent in an abandoned building fired a grenade that hissed through the air and exploded behind the last Humvee. Turret gunners returned fire, but a search afterward turned up only bullet-pocked walls.

A day earlier, the unit responded to a suicide truck bombing of an Iraqi army convoy near the outpost. That attack killed the driver of a truck carrying fuel drums and an Iraqi soldier, and wounded another five soldiers. Assailants who had waited for the blast opened fire on the Americans before fleeing the area.

"Honestly, I don't know who we are fighting," said Staff Sgt. Tim Carter, who has survived six roadside bomb attacks. "If I see them placing a roadside bomb or firing at us, then that's who we are fighting, but otherwise there is no way to tell if he is a civilian or Al Qaeda. Here, a kid can run up to shake your hand and then later throw a grenade at you."

U.S. soldiers at the outpost said some of their attackers had been as young as 11, armed with grenades or firebombs. Carter said his unit had not killed any children.

Senior U.S. commanders see their small combat outposts scattered around Mosul as the centerpiece of a strategy designed to reduce insurgent violence by having troops live within the community and gather better intelligence. Platoons rotate into the outposts on stints of several days from a large base at the city's airport.

Adrenaline flows during attacks, but life on the Rabiy outpost is also laden with unease and boredom.

The soldiers search for weapons caches, discovering one in the foundations of a half-built mosque a few blocks from the outpost. They wait for nearby mosques to begin a call to prayer over loudspeakers after nightfall, often a prelude to volleys of gunfire aimed at the outpost or other targets in what U.S. soldiers euphemistically describe as "contact."

"I think the (American) people think the war is over," Carter said. "But they don't realize the amount of contact that we receive out here."