Insurgents shot down a U.S. Apache (searchhelicopter in west Baghdad on Sunday, the military said. The fate of its two-member crew was unknown. Heavy fighting was taking place in the area for the third straight day.

"A 1st Cavalry A8-64 Apache helicopter was downed by unknown ground fire west of Baghdad at around 11:05 a.m. The condition of the crew is unknown," the spokesman said.

U.S. troops blocked traffic on the main highway out of Baghdad on the western edge. Large palls of black smoke were seen rising from the nearby area of Abu Ghraib (search), where at least four helicopters were seen hovering overhead.

Masked gunmen have been roaming the roads in west Baghdad for the past three days, attacking military fuel convoys, blowing up tanker trucks. An American civilian guarding a convoy, Thomas Hammil (search), was kidnapped in an attack Friday that also killed a U.S. soldier and an Iraqi driver.

Hammil's captors have threatened to kill him if the siege against Sunni insurgents in nearby Fallujah is not lifted.