Fallujah, Iraq ? U.S. paratroopers fired on anti-American protesters during a nighttime demonstration, and a hospital reported Tuesday that 13 Iraqis were killed and 75 wounded, including three young boys. Soldiers said armed men had mixed into the crowd and fired at them from nearby buildings.

The deaths outside a school in Fallujah, a conservative Sunni Muslim city and Baath Party stronghold 30 miles west of the capital, highlighted the tense and precarious balance as Americans try to keep the peace in Iraq.

Americans and Iraqis gave sharply differing accounts of Monday night’s shooting. U.S. forces insisted they opened fire only upon armed men — infiltrators among the protest crowd, according to Col. Arnold Bray, commanding officer of the 1st Battalion, 325 Regiment of the 82nd Airborne Division, whose troops were involved in the shooting.

“Which school kids carry AK-47s?” Bray asked. “I’m 100 percent certain the persons we shot at were armed.”

Protesters insisted their demonstration was unarmed and peaceful.

Dr. Ahmed Ghandim al-Ali, director of Fallujah’s general hospital, said the clash killed 13 Iraqis and injured about 75. The dead included three boys ages 8 to 10, he said.

Some residents put the death toll higher, at 15. Survivors said the dead were buried quickly Tuesday morning, in accord with Islamic custom.

No Americans were injured.

The shooting was the third reported fatal clash involving U.S. troops and Iraqi protesters in two weeks, underscoring the problems soldiers face as they try to switch from fighting to peacekeeping.

On April 15 and 16, Marines opened fire during angry demonstrations in the northern city of Mosul. Iraqis said 17 people were killed there, though details remained unclear and the Marines insisted they fired in self-defense.

The shootings, widely reported by Arab news media, have fueled resentment of the U.S. military weeks after the overthrow of Saddam Hussein’s regime.