Mortar rounds land outside Patrol Base Uvanni so often that soldiers inside barely lift their heads when another deafening thump disturbs the clear afternoon air. Anti-tank mines appear in...

SAMARRA, Iraq — Mortar rounds land outside Patrol Base Uvanni so often that soldiers inside barely lift their heads when another deafening thump disturbs the clear afternoon air.

Anti-tank mines appear in the dust a few yards from the entrance to the U.S. base, a brick building that was once part of a college campus. Bullets from random machine-gun fire zip past, and a sunburst of shrapnel scars remain on the outer wall from a rocket-propelled grenade.

Most U.S. bases in Iraq are regular targets of enemy fire; magnets for insurgents who are trying to hamper international reconstruction efforts. Most of the fire misses because the insurgents are not a highly trained military force. Sometimes the attacks hit hard.

The explosive attack in Mosul yesterday that caused mass casualties at a U.S. Army mess tent immediately reminded soldiers in Samarra of an attack on a nearby patrol base July 8, when a suicide bomber managed to enter the front gate and detonated a massive car bomb.

That blast killed five U.S. soldiers and four Iraqi National Guardsmen, leaving an indelible mark on the security forces who remain in this city and continue to face frequent enemy fire.

In the past few days, attacks in Samarra have become more frequent, with most focused on U.S. forces in and around their base.

Explosions occur several times a day, and mortar rounds fall within a few hundred meters of the installation. Insurgents have targeted U.S. soldiers repairing schools and Army snipers hunkered atop a historic minaret, now scarred by grenade blasts.

Senior officials here said the insurgents could be building up to a series of bigger attacks as elections approach at the end of January, and said they were particularly concerned about a large attack on a patrol base such as Uvanni, where soldiers sleep and eat in large groups.

The soldiers do what they can to protect their fortresses, using snipers and regular patrols to thwart such attacks.

Lt. Nick Kron, 25, of Richmond, Va., a tank platoon leader in Samarra, said he once discovered what appeared to be a sand-and-rock mock-up of Forward Operating Base Brassfield-Mora, north of Samarra, while on patrol nearby. Kron said the apparent reproduction included buildings and a berm built to protect against rocket attacks. Brassfield-Mora is regularly targeted by insurgent mortar fire, he said.

“It shows they’re organized and have the means to launch an attack,” Kron said.