The Organization of the Petroleum Exporting Countries (OPEC) meets in Vienna on Nov. 30 for talks on output cuts. It has sought cooperation from non-members, including Russia, but doubts remain over whether they can come to an agreement.
U.S. crude's front-month discount to the second-month widened to the biggest in nearly three months on Thursday, with traders pointing to domestic oversupply amid data showing builds in stockpiles.
Brent crude fell 61 cents at $45.75 a barrel by 2:36 p.m. (1936 GMT). U.S. West Texas Intermediate crude was down 73 cents at $44.54.
The U.S. Energy Information Administration on Wednesday reported a 2.4 million-barrel rise in domestic crude inventories to 485 million barrels last week.
That depressed the market even as stockpiles at the U.S. delivery hub for crude futures in Cushing, Oklahoma dropped by 663,916 barrels for the week, according to traders, citing energy monitoring service Genscape.
The International Energy Agency (IEA) said on Thursday the global market will remain in surplus unless OPEC can reach an agreement at its Nov. 30 meeting.
"If the supply surplus persists in 2017, there must be some risk of prices falling back," the IEA said in its monthly report.
Prices will likely rebound, at least temporarily, in the coming days and may go above $50 a barrel as traders cling to the hope of an OPEC deal, said Fawad Razaqzada, analyst at Forex.com.