OPEC on May 11 sharply raised its forecast for oil supply from non-member countries in 2017 as higher prices encourage U.S. shale drillers to pump more, hampering OPEC efforts to clear a glut and support prices by cutting its output.

In a monthly report, OPEC revised up its estimate of oil supply growth from producers outside the group this year to 950,000 barrels per day (bbl/d), up from a previous forecast of 580,000 bbl/d.

OPEC is curbing its output by about 1.2 million barrels per day (bbl/d) from Jan. 1 for six months, the first reduction in eight years, to clear excess supply. Russia and 10 other non-OPEC producers agreed to cut half as much.

The report will add to a debate about the effectiveness of the supply cut pact, which is expected to be extended when producers meet later this month. While oil prices have gained support, higher rival supply is limiting further gains and an inventory glut has proved slow to shift.

"U.S. oil and gas companies have already stepped up activities in 2017," OPEC said in the report. "U.S. tight crude output is expected to rise rapidly and increase by 600,000 bbl/d in 2017," OPEC said, using anther term for shale.

Oil prices pared gains on May 11 after the report was released to trade below $51/bbl, below the $60 level that top OPEC producer Saudi Arabia would like to see. Prices are still up from about $48 a year ago.

In the report, OPEC pointed to continued high compliance by its members with the supply cut deal and said oil stocks in industrialized nations fell in March—although they are still 276 million bbl above the five-year average.

Supply from the 11 OPEC members with production targets under the accord—all except Libya and Nigeria—fell to 29.674 million bbl/d last month, according to figures from secondary sources that OPEC uses to monitor output.

That means OPEC has complied 111% with the plan, according to a Reuters calculation, up from an estimate in March of 104%. OPEC did not publish a compliance number.