BAGHDAD -- The monthly U.S. toll in Iraq fell to its lowest point since the war began, with 10 American deaths reported as July drew to a close Thursday.

Iraqis also are dying far less with the war in its sixth year. July saw the lowest civilian toll since December 2005, though a series of suicide bombings this week and rising ethnic tensions in northern Iraq reflect the fragility of the security successes.

An Associated Press tally shows that at least 510 Iraqi civilians and security force members were killed in July, a 75 percent drop from the 2,021 deaths in the same period last year as the surge, a U.S. troop buildup aimed at quelling rampant Sunni-Shiite violence, neared its peak.

The drastic decline in violence in the past year has led to increasing optimism among American commanders, who have been wary of declaring success after past lulls proved short-lived.

It also is an issue in the U.S. presidential campaign.

"The progress is still reversible," President Bush said Thursday in Washington. But he said a new "degree of durability in gains" should permit him to announce further U.S. troop reductions later this year.

The last of five combat brigades sent as part of the surge returned home in July, leaving about 145,000 U.S. troops in Iraq. That is still higher than the roughly 130,000 to 135,000 who were there before the troop increase.

But the American soldiers appear to be taking on more of a peacekeeping role after many Sunni and Shiite extremists agreed to stop fighting.

The U.S. military has pointed to a Sunni revolt against al-Qaida in Iraq and a truce by anti-American Shiite cleric Muqtada al-Sadr as playing a large part in the drop in violence, along with the troop buildup and improvements in training Iraqi security forces.

By Kim Gamel