OPEC raised its forecast of oil supplies from non-member countries in 2017 as new fields come online and U.S. shale drillers prove more resilient than expected to cheap crude, pointing to a larger surplus in the market next year.

Demand for crude from OPEC will average 32.48 million barrels per day (MMbbl/d) in 2017, OPEC said in a monthly report on Sept. 12. That is down from the previous forecast of 33.01 MMbbl/d.

The prospect of a larger surplus than expected adds to the challenge of OPEC and non-members such as Russia, who are making a renewed attempt to restrain supplies. Oil is trading at $47/bbl, half its level of mid-2014, as a supply glut that OPEC hoped cheap oil would banish sticks around.

OPEC revised up its 2016 and 2017 non-OPEC supply forecasts, citing factors including the start up of Kazakhstan's Kashagan oilfield and a lower-than-expected decline in U.S. shale output, and said the immediate outlook was for more production.

"It is expected that there will be higher non-OPEC production in the second half of 2016 compared to the first half," OPEC said in the report.

OPEC expects non-OPEC supply to rise by 200,000 bbl/d in 2017, vs. a previously forecast 150,000 bbl/d decline. The revision is mostly due to Kashagan, OPEC said, as the long-delayed giant field finally starts up.

On top of that, the forecast for this year was revised up by 180,000 bbl/d.

OPEC itself kept output near a multi-year high in August, pumping 33.24 MMbbl/d, according to figures OPEC collects from secondary sources, down 23,000 bbl/d from July's figure, the report said.

The July figure is the highest since at least 2008, according to a Reuters review of past OPEC reports.