A reduction would signal the administration's confidence in progress in the country. On Friday, however, Casey said that war planners would take the recent violence as "certainly something that we will consider in our decisions."
Without touching on the issue of troop reduction, Khalilzad described a highly combustible atmosphere in Iraq that dates at least to the polarizing Dec. 15 legislative elections, which handed Shiites a dominant role in the government.
"Right now there's a vacuum of authority, and there's a lot of distrust," said the diplomat, who is among the architects of the U.S. plan to reshape the political balance of the Middle East after the Sept. 11 attacks.
The Samarra bombing and the subsequent outbreak of violent reprisals by Shiites against Sunni Muslims demonstrated that insurgents fully understand Iraq's fragility and will seek to exploit it, Khalilzad said.
"It indicates that they recognize this vulnerability of Iraq or this problem in Iraq, which they have tried to fan," he said. "There is a concerted effort to provoke civil war. The guys who want to start a civil war are there looking or considering other things they could do."
Khalilzad, who is actively and publicly involved in Iraq's government talks, repeated his weeks-long assertion that the best way to prevent civil war or large-scale sectarian violence is to form a government drawing from all of Iraq's disparate groups as a way "to build trust and narrow the fault line that exists" between Shiites and Sunnis.
"Once a national unity government is formed, the effort to provoke a civil war will face a huge obstacle," he said.
Shiite leaders loudly objected last week to Khalilzad's involvement in government talks, saying he was improperly taking the side of the Sunni minority.
"I have gotten some negative reaction," Khalilzad said, adding that he had not tried to intervene on the Sunni side. He said he had called for nonsectarian figures to run key ministries. "Sectarian Sunnis are as bad as sectarian Shias," he said.
In any case, Khalilzad said the U.S. has little choice but to maintain a strong presence in Iraq -- or risk a regional conflict in which Arabs side with Sunnis and Iranians back Shiites, in what could be a more encompassing version of the 1980s Iran-Iraq war, which left more than 1 million dead.
