By Christian Berthelsen 

Oil prices slipped Friday amid increasing global supplies and easing geopolitical tensions, while markets awaited macroeconomic signals from the central banker retreat in Jackson Hole, Wyo.

Light, sweet crude for October delivery fell 78 cents, or 0.8%, to $93.18 a barrel on the New York Mercantile Exchange. Brent futures on the ICE Futures Europe exchange were down 23 cents, or 0.2%, at $102.40 a barrel.

The first tanker of crude from Libya's revived production left port and was looking for a buyer, Energy Market Institute analyst Dominick Chirichella said in a note. The Italian tanker Maria Bottiglieri was expected to be loaded with 600,000 barrels from Libya's Es Sider terminal, the country's largest.

Meanwhile, there was little news of advances by Islamist rebels against oil facilities in Iraq, and tensions eased in Ukraine as a humanitarian-aid convoy from Russia was allowed to deliver aid to civilians caught in rebel-held territories.

Fed Chairwoman Janet Yellen was expected to make remarks from Jackson Hole at 10 a.m. EST. "The pre-weekend softness appears related to caution ahead of today's main event that is expected to be provided by Janet Yellen's comments," research consultancy Ritterbusch and Associates said in a note.

Analysts said oil's drop may also have been prompted by a strengthening dollar in anticipation of Fed comments; oil often moves inversely to the dollar.

September gasoline futures were down 0.8 cent at $2.7395 a gallon. September diesel was flat at $2.8374 a gallon.

Eric Yep and Francesca Freeman contributed to this article.

Write to Christian Berthelsen at christian.berthelsen@wsj.com

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