A US army helicopter crashed into a riverbank near Tikrit, killing at least four U.S. soldiers, the military said. Another American was killed and nine were wounded in attacks in the northern city of Mosul. The military said preliminary reports showed that four on board the Black Hawk helicopter were killed, although a U.S. officer on the scene said that all six on board were dead. It was not immediately clear whether the chopper was brought down by hostile fire or a mechanical failure, a spokesman said. Separately, guerrillas attacked a convoy in the eastern part of Mosul, 250 miles north of Baghdad, with rocket-propelled grenades and small-arms fire Friday morning. The military said one U.S. soldier died and six others were wounded in the clash. Three others were injured later Friday when a roadside bomb exploded near the downtown Mosul Hotel, which is now used as a military barracks. The spate of attacks in the past week in Mosul, Iraq's third-largest city, has raised concerns among occupation commanders that the resistance is spreading into that region from its main stronghold in the so-called Sunni Triangle, to the west and north of Baghdad. The U.S.-led coalition occupying Iraq suffered more casualties Thursday, with Poland recording its first combat death when Maj. Hieronim Kupczyk, 44, was killed in an ambush south of Baghdad. The Pentagon, meanwhile, announced that one of the soldiers wounded in Sunday's downing of an Army Chinook helicopter died Thursday at a medical facility in Germany, raising the death toll to 16. Twenty-six others were injured. Elsewhere, a U.S. soldier was killed Thursday when his truck hit a land mine near the Husaybah border crossing point with Syria 195 miles northwest of Baghdad, the U.S. military said. **PHOTO CAPTION*** A video grab image shows helicopters hovering over the crash site of a U.S. helicopter near the city of Tikrit, Iraq, Nov. 7, 2003. (Reuters TV/Reuters)