New York's main contract, light sweet crude for delivery in November shed 72 cents to $91.14 a barrel and Brent North Sea crude for November delivery fell 58 cents to $114.04.
Crude prices were hit by the "IEA reducing its forecast for global demand in 2012 and 2013," IG Markets said in a report.
"Data released last week also showed that crude supplies rose in the US and this seems to be a trend that may continue... while the slow growth will mean muted demand for the black gold continues," it stated.
The IEA on Friday said it expected global demand for oil through 2016 to grow by half a million barrels a day less than previously estimated.
It said in its medium-term report that the oil market was now at a "crossroads" and that its outlook was "against the backdrop of sluggish economic growth and increasing energy efficiency," particularly in advanced economies.
In the US, official data released last week showed crude stockpiles in the world's largest oil consumer rising by 1.7 million barrels in the week ending October 5, far more than analyst projections of an 800,000 barrel increase.
Rising inventories indicate weaker energy demand in the world's biggest economy.