Oil prices fell by almost 4% to below $US67 per barrel on Monday.
The World Bank earlier said prospects for the global economy remained "unusually uncertain" as it cut 2009 growth forecasts for most economies, adding to concerns of a slower turnaround.
US crude for July delivery, which expires on Monday, traded down $US2.62 to settle at $US66.93 per barrel.
Brent crude lost $US2.21 to settle at $US66.98 per barrel.
