By Daniel Huang 

U.S. oil prices fell to their lowest in almost two months on Friday as signs of abundant supplies spurred investors to resume selling.

Light, sweet crude for August delivery fell $2.10, or 2%, to $100.83 a barrel, the lowest closing price since May 12 on the New York Mercantile Exchange. Prices had climbed Thursday after falling for nine straight sessions, but Friday's drop erased those gains. Brent crude for August fell $2.01, or 1.8%, to $106.66 a barrel on the ICE Futures Europe exchange.

Forecasts of additional barrels entering the market from Libya have triggered expectations of a bloated global supply. In the U.S., government data released earlier in the week revealed an increase in the amount of oil stored at a key U.S. hub, also putting downward pressure on prices. Friday's price drop was the largest since April 22.

Technical factors were also at play. Prices fell below the 100-day moving average during the session, which accelerated the decline, said Bob Yawger, director of the futures division at Mizuho Securities in New York.

One of the most widely watched indicators in the market, the 100-day moving average, is "especially important for long traders," Mr. Yawger said. "Once prices fell below the number, that's as much pain as they can take."

A report by the International Energy Agency, pointing to rising future demand for crude, did little to brake the slide in oil prices. The IEA forecast oil-demand growth at 1.4 million barrels a day next year, compared with 1.2 million barrels a day this year. The estimate came a day after the Organization of the Petroleum Exporting Countries also predicted improved demand growth for 2015.

Front-month August reformulated gasoline blendstock, or RBOB, settled 4.91 cents lower at $2.9085 a gallon on the Nymex. August diesel lost 3.24 cents to end at $2.8609 a gallon.

Write to Ben Winkley at ben.winkley@wsj.com

Corrections & Amplifications

The price of global benchmark Brent is on track for a third successive week of losses since a recent peak of $115.71 on June 19. An earlier version of this article incorrectly said the peak was on July 19.

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