US benchmark West Texas Intermediate for delivery in October dropped USD 1.44 to USD 44.61 a barrel compared with the close on Friday.
Brent North Sea crude for October was down USD 1.64 to USD 47.97 in late London deals.
Trading volumes were down owing to Labour Day in the United States, helping to exaggerate the day's losses according to dealers.
All eyes were on the United States, the world's biggest consumer of oil, with a decision on interest rates set to influence financial markets whatever the outcome of the Fed's policy meeting next week.
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Analysts said dealers are awaiting a slew of global economic data this week for clues about crude demand, with ample oil supplies boosted by relentless US and OPEC production.
The Chinese government is scheduled to release monthly trade and inflation figures, as well as industrial output, fixed-asset investment and retail sales in the coming days.
Oil prices have fluctuated wildly in recent weeks on uncertainty about Fed monetary policy as well as worries about the growth in world number-one energy consumer China.