Production slipped 75,000 barrels, or 0.3%, to an average 29.15 million barrels a day, the lowest level since January, according to the survey. Output by members with quotas, all except Iraq, dropped 5,000 barrels to 26.805 million, 1.96 million above their target.
Iraqi output dropped 70,000 barrels, or 2.9%, to 2.345 million this month, the biggest decrease in OPEC. It was the lowest level since April. The Persian Gulf nation was the group's third-largest producer in August.
"This shows Iraq is still a risky place to do business," said Rick Mueller, director of oil markets at Energy Security Analysis Inc. in Wakefield, Massachusetts. "The events of the past week and drawdown of the U.S. presence will add to the uncertainty."
Iraqi oil exports by pipeline from the northern Kirkuk fields to the Turkish port of Ceyhan were halted from Aug. 20 to Aug. 26 after a bombing stopped the flow of crude.
The last U.S. combat unit left Iraq on Aug. 18, seven years after the invasion that ousted Saddam Hussein. Fewer than 50,000 U.S. troops remain in the country, mainly to train Iraqi forces. On Aug. 25, Iraqi cities were hit in bombings that targeted the police, killing at least 37 people and injuring 126.