OPEC on Sept. 12 further trimmed its forecast for 2019 global oil demand growth and said the risk to the economic outlook was skewed to the downside, adding a new challenge to the group's efforts to support the market next year.

In a monthly report, OPEC said world oil demand next year would rise by 1.41 million barrels per day (bbl/d), 20,000 bbl/d less than last month and the second consecutive reduction in the forecast.

The report provides further indication the rapid oil demand that helped OPEC and allies get rid of a supply glut will moderate in 2019. This means there will be less strain on other producers in making up for supply losses in Venezuela and Iran as renewed U.S. sanctions kick in.

"Rising challenges in some emerging and developing economies are skewing the current global economic growth risk forecast to the downside," OPEC said in the report.

"Rising trade tensions, and the consequences of further potential monetary tightening by G4 central banks, in combination with rising global debt levels, are additional concerns."

Crude edged lower after the OPEC report was released, trading just above $79 a barrel. Prices have slipped since topping $80 this year for the first time since 2014 on expectations of more supply and weaker demand.

OPEC and a group of non-OPEC countries agreed on June 22-23 to return to 100% compliance with oil output cuts that began in January 2017, after months of underproduction by Venezuela and others pushed adherence above 160%.

In the report, OPEC said its oil output rose in August by 278,000 bbl/d to 32.56 million bbl/d, following the June deal. The biggest rise came from Libya, which is exempt from the agreement. This helped offset declines in Venezuela and Iran.

This means compliance with the original supply-cutting deal has increased to 133%, according to a Reuters calculation, meaning members are still cutting more than promised. The original figure for July was 126%.

The August production rate is lower than the average demand for OPEC crude in 2018 and considerably more than will be needed next year as rivals such as the U.S. expand supplies.

OPEC said the world will need 32.05 million bbl/d from its 15 members in 2019, unchanged from last month. This suggests there will be a 500,000 bbl/d surplus in the market should OPEC keep pumping the same amount and other things remain equal.

The higher prices that have followed the OPEC-led deal are still prompting more growth in rival supply. OPEC said it expects non-OPEC production to expand by 2.15 million bbl/d next year, 20,000 bbl/d more than forecast last month.