The announcement appeared aimed at allaying fears that an uprising by militiamen loyal to radical Shiite Muslim cleric Muqtada Sadr could unravel security gains since 28,500 additional American troops were deployed in Iraq in a buildup that reached its height in June.

Navy Rear Adm. Patrick Driscoll, a military spokesman, credited the decrease to a series of operations launched by the Iraqi government in the last two months to extend control over parts of the country that have been under the sway of armed Sunni Arab and Shiite militants. They include crackdowns in the southern oil hub of Basra, the northern city of Mosul and Baghdad's Sadr City district.

The late March operation in Basra triggered a fierce backlash by Sadr's militiamen in Sadr City and across the overwhelmingly Shiite south, drawing in British and American forces.

The number of attacks nationwide rose to about 850 in the week that the Basra crackdown began, according to the military's chart. The figure has ebbed and flowed since then.