U.S. and Iraqi officials have hailed the killing of Zarqawi, a Sunni Arab like most guerrillas in Iraq, as a major blow to al Qaeda militants while cautioning it will not end bloodshed.

"There are going to be tough days ahead, and more sacrifice for Americans, as well as Iraqis," Bush told U.S. troops.

"Our military will stay on the offensive. We will continue to hunt down people like Mr. Zarqawi, and bring them to justice," he said to applause.

Iraqi officials billed Wednesday's clampdown as one of the biggest such operations in the last three years.

"It is an operation to step up pressure on al Qaeda in Baghdad," national security adviser Mowaffaq al-Rubaie told state television on Tuesday.