Washington - A nearly completed high-level U.S. intelligence analysis warns that unresolved ethnic and sectarian tensions in Iraq could unleash a new wave of violence, potentially reversing the major security and political gains achieved over the last year.

    U.S. officials familiar with the new National Intelligence Estimate said they were unsure when the top-secret report would be completed and whether it would be published before the Nov. 4 presidential election.

    More than a half-dozen officials spoke to McClatchy on condition of anonymity because NIE's, the most authoritative analyses produced by the U.S. intelligence community, are restricted to the president, his senior aides and members of Congress except in rare instances when just the key findings are made public.

    The new NIE, which reflects the consensus of all 16 U.S. intelligence agencies, has significant implications for Republican John McCain and Democrat Barack Obama, whose differences over the Iraq war are a major issue in the presidential campaign.

    The findings seem to cast doubts on McCain's frequent assertions that the United States is "on a path to victory" in Iraq by underscoring the deep uncertainties of the situation despite the 30,000-strong U.S. troop surge for which he was the leading congressional advocate.

    But McCain could also use the findings to try to strengthen his argument for keeping U.S. troops in Iraq until conditions stabilize.

    For Obama, the report raises questions about whether he could fulfill his pledge to withdraw most of the remaining 152,000 U.S. troops - he would leave some there to deal with al-Qaeda and to protect U.S. diplomats and civilians - within 16 months of taking office so that more U.S. forces could be sent to battle the growing Taliban insurgency in Afghanistan.

    Word of the draft NIE comes at a time when Iraq is enjoying its lowest levels of violent incidents since early 2004 and a 77 percent drop in civilian deaths in June through August 2008 over the same period in 2007, according to the Defense Department.

    U.S. officials say last year's surge of 30,000 troops, all of whom have been withdrawn, was just one reason for the improvements. Other factors include the truce declared by anti-U.S. cleric Muqtada al Sadr, the leader of an Iran-backed Shiite Muslim militia; and the enlistment of former Sunni insurgents in Awakening groups created by the U.S. military to fight al Qaida in Iraq and other extremists.

    The draft NIE, however, warns that the improvements in security and political progress, like the recent passage of a provincial election law, are threatened by lingering disputes between the majority Shiite Arabs, Sunni Arabs, Kurds and other minorities, the U.S. officials said.

    Sources of tension identified by the NIE, they said, include a struggle between Sunni Arabs, Kurds and Turkmen for control of the oil-rich northern city of Kirkuk; and the Shiite-led central government's unfulfilled vows to hire former Sunni insurgents who joined Awakening groups.

    A spokesman for Director of National Intelligence Mike McConnell, whose office compiled the estimate, declined comment, saying the agency does not discuss NIE's.

    The findings of the intelligence estimate appear to be reflected in recent statements by Army Gen. David Petraeus, the former top U.S. commander in Iraq, who has called the situation "fragile" and "reversible" and said he will never declare victory there.

    Secretary of State Condoleezza Rice echoed that tone on Monday during a State Department awards ceremony for Petraeus and U.S. Ambassador to Iraq Ryan Crocker.

    "Ladies and gentlemen, nothing is certain in this life. And success in Iraq is not a sure thing," Rice said in an uncharacteristically downbeat comment.

    The NIE findings parallel a Defense Department assessment last month that warned that despite "promising developments, security gains in Iraq remain fragile. A number of issues have the potential to upset progress."

    Trouble spots include whether the former Sunni insurgents, also known as the Sons of Iraq, find permanent employment; provincial elections scheduled for January; Kirkuk's status; the fate of internally displaced people and returning refugees; and "malign Iranian influence," the unclassified Pentagon report said.

    The intelligence agencies' estimate also raises worries about what would happen if Sadr, the anti-U.S. cleric, attempts to reassert himself, according to senior intelligence officials familiar with its contents.

    If Sadr abandons his cease-fire, it is unclear whether his former followers would rejoin his cause or whether his movement is permanently fractured, and thus harder to control.

    The embattled Sons of Iraq program may prove to be the ultimate challenge to sustained stability in Iraq. The U.S. program to pay mostly Sunni former insurgents to protect their neighborhoods or in some cases to stop shooting at Americans is now moving into the hands of the Shiite-led government.

    Many of the roughly 100,000 men of the mostly Sunni paramilitary groups have fled to Syria, while others remain in Iraq, worried that the Shiite government will disband and detain the men. The U.S. military has promised not to abandon the men, of whom about 54,000 were transferred to Iraqi government control this month.

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    (Leila Fadel contributed from Baghdad.)