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 in 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 on condition of anonymity because NIEs - 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 on 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 troop surge for which he was the leading congressional advocate.

But McCain also could 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 within 16 months of taking office so more U.S. forces could be sent to battle the growing Taliban insurgency in Afghanistan.

Word of the draft NIE comes as Iraq is enjoying its lowest levels of violent incidents since early 2004 and a 77 percent drop in civilian deaths in June through August from the same period in 2007, according to the Pentagon.

U.S. officials say last year's surge of 30,000 troops, all of whom have been withdrawn, was just one reason for the improvements.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

But the draft NIE warns that the improvements in security and political progress, such as 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.

A spokesman for Director of National Intelligence Mike McConnell, whose office compiled the estimate, declined to comment, saying the agency did not discuss NIEs.

The findings of the intelligence estimate appear to be reflected in recent statements by Army Gen. David H. Petraeus, the former top U.S. commander in Iraq, who has called the situation "fragile" and "reversible."

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

"Nothing is certain in this life," Rice said. "And success in Iraq is not a sure thing."

The NIE findings parallel a Defense Department assessment last month that warned that despite "promising developments, security gains in Iraq remain fragile."