A U.S. soldier was killed in a mortar assault on a military base north of Baghdad, the U.S. Army said Saturday, and a top commander warned attacks by Iraqi resistance fighters were growing more sophisticated. Several mortars were fired at a base near the town of Balad, about 80 km (50 miles) north of the capital Baghdad, Friday evening, killing one soldier and wounding two others, all from the 4th Infantry Division, a U.S. Army spokesman said. "The base came under mortar attack and one soldier was killed by shrapnel, while two others were wounded," Sergeant Robert Cargie of the 4th Infantry Division told reporters. He said six people were detained in connection with the assault. The two wounded soldiers, who were taken to a nearby field hospital, were in a stable condition. News of the attack came a day after fighters shot down an OH-58 Kiowa Warrior helicopter outside the town of Falluja, west of Baghdad. It was the sixth U.S. helicopter to be brought down by Iraqi insurgents since October. A policeman who witnessed the crash said the chopper was hit by a missile before falling to the ground. One pilot was killed and the other wounded. It was the first time a Kiowa, a nimble craft used for observation, had been shot down in Iraq. U.S. forces quickly surrounded the crash site to keep witnesses and journalists at bay. American soldiers detained three Iraqis working for Reuters as they covered the aftermath of the crash. A Reuters driver working with the three said they had earlier been fired on by U.S. troops as they filmed a checkpoint close to the site. U.S. Army spokesman Brigadier General Mark Kimmitt separately told a news conference in Baghdad that resistance fighters posing as journalists had fired on U.S. soldiers guarding the area and four were later detained. **Fighters more Sophisticated*** Kimmitt also said the number of resistance attacks was falling but warned they were getting more sophisticated. "For what reason, we don't know, but they are getting a little more sophisticated of late." In Kirkuk, 250 km (155 miles) north of Baghdad, ethnic tensions that have flared on several occasions in recent days, resulting in the deaths of at least six Iraqis, appeared to abate as U.S. forces and local authorities imposed a curfew. **PHOTO CAPTION*** An Iraqi Civil Defense Corps member patrols while Iraqis wait in line to buy cooking gas at a propane gas station in Baghdad, Saturday Jan. 3, 2004. (AP Photo/Muhammed Muheisen)