Bush Says Iraqis Face Moment of 'Choosing'

"This is an extremely hard and extremely delicate moment, obviously, for the Iraqis," Rice said. There are "heightened sensitivities and people's nerves are a bit on edge when you have this kind of strike against Iraq unity."

The United States has tried to bring various Iraqi factions together to form a unity government, but the widening violence makes that effort even more urgent.

White House national security adviser Stephen J. Hadley said the administration is hoping that the current crisis will help the various Iraqi factions "to see the importance for them to come together and to accelerate the establishment of an unity government."

"The question is, can this crisis also be an opportunity for bringing some Iraqi political leaders to their senses and encouraging cooperation," said James A. Phillips, of the conservative Heritage Foundation. "In a logical world that may be the case, but this is the Middle East."

The violence in Iraq this week has stirred debate about whether the U.S. military should continue to consider troop withdrawal plans. Although commanders hope to gradually draw down the 133,000 U.S. forces in Iraq, defense officials are basing decisions on troop levels on the conditions on the ground, which now appear to be worsening.

While the U.S. military avoided massive displays of force on the streets of Iraq over the past few days -- preferring to let Iraq's developing security forces take the lead in quelling violence -- defense and Iraq experts said that a withdrawal of troops in the near term could spell disaster.

A. Heather Coyne, who just returned from Baghdad, where she was the chief of party for the U.S. Institute of Peace, said the United States should avoid trying to take military control of the situation and instead should focus on building the Iraqi government's authority and providing basic services. She said, however, that troops are needed to give people a sense of stability and that such stability might not exist on its own for several years.

"A precipitous withdrawal will cause chaos," said Coyne, who spent three years in Iraq, initially as a civil-military affairs officer.

Although Coyne and others believe Iraq has been on the brink of civil war for some time, they emphasize that has not happened.

"The terrorists would like to see this break out in civil war, but I don't think the people are going to allow that to happen," Army Col. Jeffery J. Snow, commander of the 1st Brigade, 10th Mountain Division in Baghdad, said in a videoconference. Snow said Iraqi security forces are developing well but "it is important not to be unrealistic." He added: "The defeat of the counterinsurgency is going to take time, and the growth of a new army cannot be instantly realized."

Though some members of Congress have called for an immediate withdrawal of U.S. troops and a complete handover to the more than 230,000 Iraqi troops, some say such a move could cause Iraqi society to implode because the security situation is so tenuous.

"Withdrawal right now would be irresponsible," said Michael Rubin, a resident scholar at the American Enterprise Institute who suggested that raising U.S. forces back to about 160,000 troops -- the level in Iraq last fall -- might be prudent.

"Democrats, Republicans, military officials and Iraqis will all acknowledge that the greatest threat is creating a vacuum, and that's what the insurgents want. Insurgents need to realize that if there is a civil war, they are going to be on the losing end of it," Rubin said.

Staff writer Glenn Kessler in Shannon, Ireland, contributed to this report.