Oil traded near $71 a barrel as rising stockpiles and signs of slower demand growth countered concerns Iranian supplies will soon be curbed by sanctions.
Futures dropped as much as 0.7 percent in New York.
The International Energy Agency trimmed estimates for demand growth, and boosted those for supply, as prices have jumped to three-year highs. US industry data was said to show crude inventories rose last week.
The European Union and Iran are identifying ways to maintain oil shipments from Iran, easing fears of a supply drop-off.
“The market is less tight than hitherto assumed,” said Eugen Weinberg, head of commodities research at Commerzbank AG in Frankfurt. “Non-OPEC supply is growing more strongly than previously anticipated.”
The Organization of Petroleum Exporting Countries and its allies have also finally succeeded in clearing a global glut, the IEA said.
Yet rising prices could be backfiring by slowing demand growth and boosting supply from countries like the US.
West Texas Intermediate crude for June delivery dropped as much as 49 cents to $70.82 a barrel on the New York Mercantile Exchange, and traded at $71.08 as of 8:49 a.m. local time. Total volume traded was 2 percent below the 100-day average.
Brent for July settlement lost 55 cents to $77.88 a barrel on the London-based ICE Futures Europe exchange, after adding 20 cents on May 15. The global benchmark crude traded at a $6.75 premium to WTI for July.
Futures for September delivery on the Shanghai International Energy Exchange were little changed at 473.1 yuan a barrel after gaining 1.6 percent on May 15.
The American Petroleum Institute was said to report US crude stockpiles climbed 4.85 million barrels last week, and that inventories at the key pipeline and storage hub Cushing, Oklahoma, rose 62,000 barrels. Gasoline stocks declined by 3.37 million barrels, according to the API.
The nationwide inventory estimate contrasts with forecasts in a Bloomberg survey that show stockpiles declined by 2 million barrels. Government data was expected on Wednesday.
Investors are keeping a close watch on how much crude will end up being removed from the global market, and whether other major producers will ramp up output to fill any gaps.