A group of 18 mostly Democratic U.S. senators on Friday urged the Obama administration to stop Royal Dutch Shell's preparations for oil exploration in the Arctic, saying the region has a severely limited capacity to respond to accidents.

Oil prices slipped on Friday as worries over the impact of war in the Middle East on crude supplies were outweighed by reports of profit-taking ahead of a long weekend.

Oil prices rebounded on Wednesday, with U.S. crude snapping a five-day decline, after another weekly inventory draw but gains were still limited by a huge supply overhang and concerns about a stronger dollar.

Crude oil prices bounced back on Wednesday from steep falls in the previous session as industry data showed that U.S. crude stocks fell more than expected last week.

Brent crude oil fell below $66 a barrel on Tuesday as the dollar strengthened and on evidence of ample supplies of Middle Eastern oil despite wars in northern Iraq, Syria and Yemen.

Oil slipped on Monday as a rallying dollar and concerns of growing oversupply weighed on the market after Saudi Arabia reported its highest crude exports in nearly a decade.

Goldman Sachs has cut its long-term crude oil price forecasts and recommended investors sell shares in two major oil companies, saying that improved U.S. shale efficiency and higher production from OPEC will more than cover future demand.

Oil prices rose on Monday after Islamic State militants said they had seized control of the key city of Ramadi in western Iraq, raising fears of deeper turmoil in the oil-producing country.

The average price of a gallon of regular-grade gasoline in the United States rose 22 cents over the past three weeks to $2.82, according to a Lundberg survey released on Sunday.

Brent crude oil steadied below $67 a barrel on Friday after reports that a growing supply glut is boosting inventories worldwide.