Brent crude oil prices fell below $US100 a barrel for the first time in five months on Tuesday.
The drop was due to expectations that OPEC would leave formal output targets unchanged and came as a threat by Hurricane Ike to oil rigs in the Gulf of Mexico, receded.
London Brent fell $US3.88 to $US99.56 a barrel - the first time since 2 April - while the US benchmark fell $US3.55 to $US102.79 a barrel.
OPEC ministers are due to meet in Vienna this week.
Oil has fallen in price by nearly 30% from a record of $US147.11 a barrel on 11 July.