Oil prices have fallen below $US50 per barrel for the first time since May 2005 amid fears of a recession and expectations that demand will drop.
US light sweet crude fell on Thursday to $US49.75/b. In London, Brent crude fell to $US48.90/b.
Oil is now at prices last seen more than three years ago and around two-thirds cheaper since reaching a record $US147.27 per barrel on 27 July.
Deutsche Bank said on Wednesday that oil could fall to as low as $US40 per barrel next year.
Members of the Organization of the Petroleum Exporting Countries last month agreed to reduce output by 1.5 million barrels per day.
OPEC ministers are to hold informal talks on 29 November in Cairo.