West Texas Intermediate futures for October delivery fell to $44.59 (-1.39%), while Brent crude dropped to $49.20 (-1.70%). Some analysts said that prices declined amid profit taking after both crudes finished last week with gains.

Investors are still focused on Fed interest rates. Higher rates would make this dollar-denominated commodity more expensive for holders of other currencies.

Market participants are also eyeing the result of planned United Nations-brokered talks later this week between Libya's warring factions aimed at forming a unity government. Yesterday the Tripoli-based central bank reported that Libya recorded a budget deficit of $3.3 billion in the first seven months of 2015 as oil prices and production fell.