Brent crude and WTI prices slipped on Tuesday after the International Energy Agency (IEA) warned that oil prices may decline as stocks continue to increase this year.
U.S. crude futures closed down $2.84, at $50.02 a barrel after hitting a session low of $49.91 a barrel.
Brent crude fell about $2 to $56 a barrel, ending a three-day rally. The benchmark gained more than 9 percent last week, its biggest weekly rise since February 2011.
Oil stocks held by countries in the Organisation for Economic Cooperation and Development may come close to the all-time high of 2.83 billion barrels in the middle of 2015, said the IEA, which advises the West on energy policy.
"Despite expectations of tightening balances by end-2015, downward market pressures may not have run their course just yet," the IEA said in a monthly report.
Vitol chief Ian Taylor, speaking at the International Petroleum Week industry conference, said he expected a "dramatic" build in oil stocks over the next few months but that supply and demand in the oil market would move into balance in the second half of this year.