U.S. Defense Secretary Donald Rumsfeld downplayed the diplomat's comments, saying "natural historical differences" among Iraqis are posing a built-in risk of civil war.
Khalilzad said Iraq had pulled back from the brink after a deadly surge of sectarian violence after the February 22 bombing of a Shi'ite Muslim shrine in Samarra.
But he said Iraq will be "really vulnerable" if a similar incident occurs again, and the "potential is there" for sectarian violence to become full-blown civil war.
He said the United States must keep a strong presence in Iraq to avoid risks to Gulf energy supplies, and the threat that parts of Iraq would become a base for religious extremists.
Rumsfeld insisted that reports on the recent burst of violence in Iraq were exaggerated.
RFE/RL Iraq Report
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