A top U.S. military official in Iraq says insurgents have shot down a U.S. attack helicopter in Baghdad, killing its two crew members.

The Apache attack helicopter was shot down Sunday morning, west of Baghdad. Large plumes of black smoke could be seen from the area where the aircraft went down.

In Fallujah, also west of Baghdad, civilians streamed out of the besieged city, where U.S. military forces have been waging battle against Sunni Muslim fighters for the past week. In the opposite direction, trucks carrying food and water supplies drove into Fallujah. Hundreds of people have been killed or wounded in the past week.

The truce appears to be holding. However, two U.S. Marines were wounded by sniper fire Sunday, as they patrolled an industrial area of the town. Other Marines returned fire, and killed at least one of the snipers.

U.S. forces attacked Fallujah in response to the killings and mutilations of four American private security guards, who were ambushed in the town last week.

American military officials say leaders in Fallujah must hand over those responsible for the killings.

Insurgents are threatening to kill a U.S. hostage they claim to be holding, unless the military ends its siege.

Saturday, a militant group said it was holding 30 hostages including Americans, Israelis, Spanish, Bulgarians, Japanese and Koreans. The Dubai-based Arab television station, Al-Arabiya, broadcast a statement from the group, which claimed the hostages would be killed unless U.S. forces withdraw from Iraq. The group also claimed responsibility for the killings of the four American security guards in Fallujah.