Brent crude had fallen 39 cents, or 0.8 per cent, to at $50.98 per barrel by 0553 GMT (11:23 am in India), its lowest since November 30. It closed the previous session down 1.6 per cent at $51.37.
US West Texas Intermediate crude (WTI) declined 45 cents, or 0.9 per cent, to $48.04 a barrel, also the lowest since November 30.
Brent crude is set to drop for a fifth day, its biggest losing streak since a six-day slump that ended on November 4. During this streak, the price has slumped 8.8 per cent, its worst performance since the middle of June 2016.
WTI is set to decline for a sixth day, during which is has dropped 9.8 per cent.
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 US, the world's biggest oil consumer, have surged to a record.
US drillers added oil rigs for an eighth consecutive week, Baker Hughes said on Friday, as energy companies increased spending to take advantage of an earlier recovery in crude prices since the Organization of the Petroleum Exporting Countries (OPEC) agreed to cut output.
OPEC and other major oil producers including Russia reached a landmark agreement late last year to rein in production by almost 1.8 million barrels per day (bpd) in the first half of 2017.
Overshadowing the curbs, US crude inventories surged last week by 8.2 million barrels.
"With the market still digesting the big increase in inventories, oil prices are likely to remain under pressure today," ANZ bank said in a note.
Hedge funds and other money managers cut their net long positions in US crude futures and options in the week to March 7, according to data from the US Commodity Futures Trading Commission (CFTC) on Friday.
Michael McCarthy, chief market strategist at Sydney's CMC Markets, said markets this week will be dominated by expectations that the Federal Reserve is set to hike interest rates this week.
A rise in US rates would likely buoy the dollar, making greenback-denominated oil more expensive for importing countries.
"The week ahead is packed with potentially market defining releases," said Mr McCarthy. "However, the key to market performance this week is the response to the US lift in rates."