Brent crude fell to the lowest in almost 14 months after Kurdish and Iraqi forces seized control of Iraq's largest dam from Islamic State militants. West Texas Intermediate declined.

Brent slid as much as 2 percent. Kurdish forces and government anti-terrorism units took over the Mosul Dam after receiving air support from the U.S., reversing gains made by the Sunni-Muslim insurgents in the north, according to Iraqi military spokesman Qassim Ata. Futures rose the most in a month on Aug. 15 after Ukraine said its forces attacked and partially destroyed a military convoy entering the country from Russia.

"The anti-ISIS forces are gathering strength and making progress," said Michael Lynch, president of Strategic Energy & Economic Research in Winchester, Massachusetts. "The threat has peaked, and the risk premium is declining. We are selling off from Friday's Ukraine-inspired bump."

Brent for October settlement slid $1.95, or 1.9 percent, to $101.58 a barrel at 9:39 a.m. New York time on the London-based ICE Futures Europe exchange. It fell as far as $101.43, the lowest since June 26, 2013. The contract rose $1.46 on Aug. 15, the most since July 17. The volume of all futures traded was about 5.9 percent below the 100-day average for the time of day.

WTI for September delivery decreased $1.18, or 1.2 percent, to $96.17 a barrel on the New York Mercantile Exchange. Volume was 2.2 percent below the 100-day average. The October contract of the U.S. benchmark crude was at a discount of $7.70 to Brent for the same month on ICE. The spread closed at $8.21 on Aug. 15, the widest since June 24.