President Bush warned Friday that sectarian violence is confronting Iraqis with a “moment of choosing,” as administration officials pleaded with all sides of the country’s religious and ethnic divides to show restraint.

Bush said Friday that Zalmay Khalilzad, the U.S. ambassador to Iraq, had met with the leaders of an array of Iraqi factions in an effort to promote unity over reprisals.

The latest bloodshed poses a new challenge to the Bush administration’s strategy for reconstruction and eventual troop withdrawals. The sectarian attacks threaten to paralyze Iraq’s fledgling political process while prompting fears that unrest could wash into neighboring countries.

Bush and senior officials sounded a newly grim note Friday about the near-term difficulties. Several independent analysts said the attacks would raise political pressure to bring U.S. troops home, even as they underscore that Iraq is too unstable to allow a precipitous withdrawal.

Speaking in Washington to the American Legion, Bush blamed the violence on insurgents intent on disrupting Iraq’s democratic progress, and predicted the violence is likely to continue.

“The days ahead in Iraq are going to be difficult and exhausting,” the president said. Still, he pleaded for patience, saying Iraq’s leaders are committed to stopping civil strife and that the will of the moderates eventually will take hold.

More than 100 people across Iraq have been killed in clashes in attacks and counterattacks between Sunni and Shiite Muslims touched off by the bombing earlier this week of the Askariya shrine, a revered Shiite shrine in Samarra. The attacks prompted the Iraqi government to impose a curfew in Baghdad and three provinces. That led to a sharp reduction in the violence Friday, as U.S. and Iraqi troops worked to maintain the peace.

In a telephone briefing with reporters, Khalilzad called the situation “a moment of, of course, danger, but it is also a moment of opportunity.”

“In crises such as the one caused by this attack,” he said, “there is an opportunity to bring people together and to defeat goals of those who want to promote a civil war in this country.”

President Bush pauses during a lengthy address on the global war on terrorism at a gathering of the American Legion in Washington, Friday, Feb. 24, 2006. Bush says the situation in Iraq remains serious. But the president also says he's optimistic Iraqis will turn away from civil warfare in what he calls their moment

Secretary of State Condoleezza Rice, returning from a weeklong visit to the Middle East, said Arab leaders expressed fear that violence will spread.

“There is a concern that sectarian tension that outsiders are stoking in Iraq, that those same outsiders might try and stoke sectarian tensions in other parts of the region,” Rice said.

The prospect of a wider regional war erupting between Sunni and Shiite populations if civil war broke out in Iraq has been a concern of many analysts.

“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.

The violence in Iraq this week has stirred debate about whether the U.S. military should continue to consider troop withdrawal plans. While commanders hope to gradually draw down the approximately 133,000 U.S. forces now in Iraq, defense officials are determining troop levels based 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.