The incidents came as the Bush administration was extending the tours of some troops in Iraq and sending additional ones for a buildup that has met strong resistance in Congress. The administration has billed the increase as a key step toward securing the viability of Iraq's young government and bringing down the level of violence.

The third most lethal day for American soldiers in Iraq also coincided with the arrival of the 2nd Brigade of the 82nd Airborne Division, the first unit to arrive in Baghdad as part of a 21,500 troop surge that the Bush administration hopes will restore order in the country's violent capital.