Oil hovered around three-month lows today, as rising US inventories and drilling activity offset optimism over OPEC's efforts to restrict crude output.
Brent crude was down seven cents on the day, at $51.30 a barrel this afternoon, having hit a session trough of $50.85, its lowest level since November 30.
US West Texas Intermediate crude (WTI) fell 15 cents to $48.34 a barrel.
The price of oil has fallen by more than 8% since last Monday, its biggest week-on-week drop in four months, and analysts said the slide may not have much further to run.
US drillers added oil rigs for an eighth consecutive week, Baker Hughes said on Friday, lifting spending to benefit from an earlier recovery in crude prices since the Organisation of the Petroleum Exporting Countries (OPEC) agreed to cut output.
OPEC and other major oil producers including Russia reached an agreement late last year to rein in production by almost 1.8 million barrels per day (bpd) in the first half of 2017.
Although OPEC states have been complying with supply curbs, led by Saudi Arabia, it has not been enough to overshadow a rise in US inventories to a new high.