FALLUJAH, Iraq — A black-garbed Iraqi gunman slinked over a rooftop and shimmied down a palm tree, pausing for a few seconds to grab a rifle from a comrade.

A few blocks away, on another rooftop, a Marine sniper squeezed a trigger and shot the man in the leg. A second shot into his chest killed him, throwing his body out of the tree. The man became Sgt. Sean Crane's 11th kill in Fallujah.

The front lines in the siege of Fallujah are the realm of snipers, as riflemen on both sides of the fight seize the high points of the streetscape. The snipers have been operating even during an uneasy truce over the past week.

Lying flat-bellied on rooftops or leaning over rifles poking out of second-floor windows in darkened rooms, Marine snipers pick off gunmen darting across streets. And Iraqi riflemen fire at U.S. positions from buildings and mosque minarets.

Residents of Fallujah have lived in terror of the Marine snipers and have blamed them for civilian deaths, particularly during heavy fighting in the first week after the siege began April 5. Iraqis said it seemed that just stepping outside or looking out a window at the wrong time could draw sniper fire.

Haqi Ismail was shot dead by an American sniper just after leaving his house for prayers at a nearby mosque, said his cousin Ismail Hamada.

"His wife could not move forward to help him because she would have been killed, too. She stood crying as he bled to death," Hamada told The Associated Press in Baghdad, where he fled with his family.

The Marine offensive to crush Sunni insurgents in this Euphrates River city has killed five Marines and more than 600 Iraqis, mostly civilians, according to hospital sources. The push was stopped on April 9 to allow for negotiations.

But Marines continue to defend their positions, responding to fire but also attacking to break up insurgent movements that could threaten them.

On a recent afternoon, a Marine anti-armor team fired a missile that clipped off the top of minaret where troops had spotted a muzzle flash.

Crane leads a squad of Marine snipers posted along a row of houses on the city's northern edge. "If the enemy is taking to the rooftops, you want to be on high ground, too," he said.

A mound of freshly turned earth in a dark alley below his post marked a shallow grave where he buried a gunman he shot in the street below.

Crane, 30, from Fort Lauderdale, Fla., and his team spend hours scanning streets and rooftops through powerful scopes that pick up body heat, outlining the shape of a figure in darkness.

Long shots, sometimes at distances of 1,000 yards, have to be finely adjusted to account for wind, temperature, barometric pressure and distortions from sunlight, shadows and waves of heat from the ground.

The calculations have to be split-second. Snipers sometimes guess wind speed, for example, by the movement of blowing trash.

One Marine rifleman missed an insurgent sniper — considered a No. 1 target — because of poor depth perception through a high-powered scope.

The Iraqi gunman was casually walking across a rooftop, and he slowly brought a Soviet-made sniper rifle up to aim as though he were a farmer readying to take a few shots from his back porch for fun, said Sgt. Ryan Warden, 28, who was watching the man's movements.

His partner fired twice but missed. "I wanted to end his life that day, but it didn't happen," said Warden, from Birmingham, Ala. "He had no idea we were on to him."

In Fallujah, Warden had his first confirmed kill. "I thought it would feel weird, but it didn't," he said.

"It probably changed me in some way or made me appreciate life more," added Warden, who gave up a career as a model to re-enlist as a Marine Corps sniper — something his fellow riflemen tease him about.

A halt to the Marine offensive has created challenges for the team. Snipers prefer to change positions after a few shots to keep their posts secret so gunmen can't hone in on them. But now troops are prevented from advancing beyond the street that marks their front line.

That also means insurgents learn which streets to avoid.

"For the first few days, we were hitting five a day," Crane said. "The word is out. It's tapered off to ones and twos."

The longer a sniper stays put, another problem emerges: barking dogs and birds taking off at the sound of a shot can give away his position.

Before coming to Fallujah, Crane was posted in an open field in Iraq, where sheepherders would occasionally stumble upon him. Once, while aiming a rifle from a pile of rocks around his foxhole, he came face to face with the yellowish eye and wooly face of a sheep. He held still and the farmer, mumbling quietly to his flock just a few steps away, never saw him.

In Fallujah, Marine snipers set up rifles in front of small holes knocked out of walls with sledge hammers. Others hunker down at the corners of windows, where they've drawn shut curtains and positioned bookcases and other furniture to block light that might reveal their silhouette.

Iraqi gunmen are often hit in the early morning and early evening, as they travel to and from points of attack on U.S. forces. Some have done combat dive rolls across streets or hidden behind civilians to try to avoid being hit, Marines said.

During the first week of fighting, some residents reported seeing Marines firing from the tops of minarets, particularly that of al-Khulafaa Mosque in eastern Fallujah. A local cleric even issued a religious ruling allowing insurgents to shoot at minarets to down the Americans.

A 16-year-old living near al-Khulafaa, Mohannad Abdel-Rahman, went up on his roof and was shot in the head by a sniper, his relatives told AP. When his uncle went to retrieve his body, he too was killed, they said.

But Sgt. Warden says the last place he'd want to be is up in a minaret with no escape route, and as far as he knows, entering mosques is off limits for the Marine force.

"My lieutenant told me, 'I'd sure like to get you up in that mosque,' " Warden said. "But no way. If my position were located there, it would take forever to get down. With no escape route, you're screwed."