Oil fell over 2 percent on Friday, extending the week's loss to the largest in eight months, pressured by swelling storage of crude on both land and sea.
Prices slipped slightly after Baker Hughes reported the number of oil rigs operating in the United States rose for the first time in 11 weeks.
The weekly count ticked up by 2 rigs to a total of 574, compared with 1,578 at this time last year.
U.S. crude traded slightly above $40 a barrel while benchmark Brent was less than $2 from setting new 6½-year lows. The slump widened to oil products with U.S. gasoline tumbling to 10-month lows.
Oil prices have fallen in seven of the last eight sessions, with losses accelerating after U.S. government data on Thursday affirmed a seventh weekly rise in U.S. crude inventories that took stockpiles near April's record highs.
The International Energy Agency (IEA) added to the bearish sentiment on Friday, saying there was a record 3 billion barrels of crude and oil products in tanks worldwide.