KERBALA, Iraq (Reuters) - When a young Iraqi boy stooped to pick up a rocket propelled grenade off the body of a dead paramilitary, U.S. Army Private Nick Boggs made a tough call.

He unloaded machinegun fire and the boy, whom he puts at about 10 years old, fell dead on a garbage-strewn stretch of waste land.

Boggs, a softly spoken 21-year-old former hunting guide from Alaska, says he knew when he joined the army 18 months ago he might someday have to make a decision like that.

He hoped it would never come and, although he has no regrets about opening fire, it is clear he'd rather it wasn't a child he killed.

"I did what I had to do. I don't have a big problem with it but anyone who shoots a little kid has to feel something," he said after fierce weekend fighting in this Shi'ite Muslim holy city that left dozens of Iraqis and one American soldier dead.

As U.S. troops take the Iraq war out of the desert and into the main cities, they are increasingly seeing children in their line of fire.

Many are innocent civilians in the wrong place at the wrong time and military officers concede that some have may have been killed in artillery or mortar fire, or shot down by soldiers whose judgment is impaired in the "fog of war."

LEGITIMATE TARGETS

But others are apparently being used as fighters or more often as scouts and weapons collectors. U.S. officers and soldiers say that turns them into legitimate targets.

"I think they're cowards," Boggs said of the parents or Fedayeen paramilitaries who send out children to the battlefield.

"I think they thought we wouldn't shoot kids. But we showed them we don't care. We are going to do what we have to do to stay alive and keep ourselves safe."

The boy he killed was with another child of around the same age when they reached for the RPG and came under fire. Boggs thinks the second boy was also hit but other soldiers think he escaped and that he dragged his friend's dead body away.

Boggs' platoon leader, Lieutenant Jason Davis, said the young soldier struggles with what happened even if he had no choice but to shoot.

"Does it haunt him? Absolutely. It haunts me and I didn't even pull the trigger," he said. "It blows my mind that they can put their children into that kind of situation."

Although Boggs plays down suggestions he was upset by the incident, he also says his view of combat has changed since Saturday, when his platoon came under intense RPG and rifle fire from the moment they entered Kerbala until way after nightfall.

Before -- like many young soldiers -- he says he was anxious to get his first "kill" in a war. Now, he seems more mature.

"It's not about killing people. It's about accomplishing a mission...When we talk, we don't say how scared we were. But we found out how you feel when an RPG hits the wall just up from you and you think 'Damn, I could have been right there'," he said.