OPEC on Wednesday said it replaced a drop in Iranian exports as US sanctions resumed and lowered the 2019 forecast of demand for its crude, underlining its challenge to prevent a glut building even after last week's decision to trim output.

In a monthly report, OPEC said its oil output fell by only 11,000 barrels per day, month on month, to 32.97 million bpd in November, despite President Donald Trump's reimposition of sanctions on Iran, as Saudi Arabia pumped at a record rate.

The Organization of the Petroleum Exporting Countries also said 2019 demand for its crude would fall to 31.44 million bpd, 100,000 less than predicted last month, as rivals pump more and a slowing economy curbs demand growth.

OPEC and allies last week agreed to return to supply cuts in 2019, pledging to lower output by 1.2 million bpd.