At around 0800 IST, US benchmark West Texas Intermediate (WTI) for delivery in April was down 19 cents to USD 38.31 and Brent crude for May was down two cents at USD 40.37.
Tehran announced yesterday that it would join a planned meeting between producer giants only after its output has reached pre-sanction levels of 4.0 million barrels per day.
The March 20 meeting in Russia to discuss an output freeze seems increasingly unlikely if Iran does not participate in the talks, led by OPEC kingpin Saudi Arabia.
But the market has rebounded in recent weeks, with WTI touching a three-month high and Brent breaching the USD 41 mark Wednesday.
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On Friday, the Paris-based International Energy Agency said prices may have "bottomed out" after a 20-month rout.
Phillip Futures analyst Daniel Ang said selling pressure set in today as traders took profit following last week's rally.
"Traders are selling at a high level, which explains the prices dipping this morning. But some of it also comes from Iran's announcement about not cutting back oil production," he added.
Traders are also watching a meeting of US Federal Reserve policymakers tomorrow and Wednesday, with attention on whether they will announce another interest rate hike.
A rate increase is a boost to the dollar, which would make dollar-priced oil more expensive, hurting demand and prices. The Fed raised rates for the first time in nearly a decade in December.