Brent crude was down 11 cents, or 0.21 percent, at $51.26 per barrel by 0755 GMT, its lowest since Nov. 30. It closed the previous session down 1.6 percent at $51.37.
U.S. West Texas Intermediate crude (WTI) declined 19 cents, or 0.39 percent, to $48.30 a barrel, also the lowest since Nov. 30.
Brent crude is set to drop for a fifth day, its biggest losing streak since a six-day slump that ended on Nov. 4. During this streak, the price has slumped 8.5 percent, its worst performance since the middle of June 2016.
WTI is set to decline for a sixth day, during which is has dropped 9.3 percent.
The slump in prices has occurred as more rigs are deployed to look for oil in the United States and as crude inventories in the U.S., the world’s biggest oil consumer, have surged to a record.