Benchmark Brent futures were down 59 cents at $79.21 a barrel by 0922 GMT, while U.S. crude futures eased 41 cents to $71.43 a barrel.
The group previously agreed to curb their output by about 1.8 million barrels per day to boost oil prices and clear a supply glut.
The Organization of Petroleum Exporting Countries (OPEC) may decide at its meeting in June to increase oil output to make up for reduced supply from Iran and Venezuela and in response to concerns from Washington over a rally in oil prices, OPEC and oil industry sources told Reuters. US West Texas Intermediate (WTI) crude slumped $2.83, or 4 percent, to finish at $67.88 a barrel.
OPEC and allied oil producers including Russian Federation concluded that the crude market re-balanced in April, when their output cuts achieved a key goal of eliminating the global surplus. The pact began in January 2017 and is set to expire at the end of 2018.
If OPEC officials follow through at a meeting late next month, the move could modestly reduce gasoline prices for American consumers, but likely wouldn't alter the longer-term climb in fuel costs amid strong global demand, experts say. The premium of Brent to WTI hit $8.75 per barrel in post-settlement trade, its widest since March 2015.The energy ministers of Russian Federation and Saudi Arabia met in St Petersburg to review the terms of a global oil supply pact that has been in place for 17 months, ahead of a key OPEC meeting in Vienna next month.
Saudi Energy Minister Khalid al-Falih said the easing of restrictions would be gradual so as to not shock the market, noting that producing countries would soon have the capacity to liberalize supply and that this could probably happen in the second half of 2018. The decline was due to producers' greater adherence to their pledged output cuts - their compliance rate reached 152 percent in April - and to summer demand for crude and refined products, they said.
OPEC and some non-OPEC major oil producers are scheduled to meet in Vienna on June 22.
With Iran, one of OPEC's top producers, facing tough US sanctions pressure and Venezuelan output at historic lows, a source speaking to Argus said extraordinary action may be necessary to calm the hot oil market.
Investors were also looking ahead to inventory data later in the day.
Meanwhile, commercial U.S. crude inventories rose 5.8 million barrels in the week to May 18, beating analyst expectations for a drop of 1.6 million barrels, the Energy Information Administration (EIA) said on Wednesday.