Oil production from the Organization of Petroleum Exporting Countries totaled 31.26 million barrels per day in August, down 140,000 bpd from the July level of 31.4 million bpd.
This was mainly because of several member states, including Saudi Arabia, trimming output, according to a Platts survey of OPEC and oil industry officials and analysts.
"This is the first time OPEC output has fallen since February, but it shouldn't be over-interpreted," Margaret McQuaile, senior correspondent for Platts, a leading global provider of energy and commodities information, PR Newswire reported.
"The August total still puts OPEC exceeding its official ceiling by 1.26 million bpd and Saudi oil is still flowing at record levels."
The August estimate marked the first monthly fall in OPEC output since February, although the total remained more than 1 million bpd above the group's official 30 million bpd ceiling.
Despite having rolled over the ceiling, in place since the beginning of 2012, in June for a further six months, OPEC has no mechanism for enforcing it as there are no individual country quotas.
Saudi Arabia, having pumped at record levels above 10 million bpd since March, scaled back supply by 50,000 bpd to 10.4 million bpd in August.
The world's biggest producer, which drove OPEC's November 2014 decision not to reduce output despite sliding oil prices, has given no indication that it is ready to abandon its market share strategy. Smaller decreases came from Iraq, Libya, Angola and Qatar.
Libyan production remains at less than one quarter of its 1.6 million bpd capacity, with output slipping to just 360,000 bpd in August from 390,000 bpd in July.
In Iraq, exports last month were slightly down on both July and June because of a slight fall in southern exports and the ongoing restrictions on supplies via Turkey.
Earlier this week, Iranian Oil Minister Bijan Namdar Zanganeh said OPEC's current policy had proved unsuccessful in marginalizing shale oil, a key driver in climbing non-OPEC production, and that member countries should now be willing to reduce supply to boost prices.
