OPEC cut its forecast for 2019 world oil demand on Tuesday due to slowing economies and expectations of faster supply growth from rivals, underlining its challenge to prevent a surplus even as it starts new production cuts.

In a monthly report, the Organization of the Petroleum Exporting Countries lowered its forecast for 2019 economic growth and said demand for its crude would fall to 30.59 million barrels per day, 240,000 bpd less than predicted last month.

OPEC, Russia and other non-OPEC producers, an alliance known as OPEC+, agreed in December to reduce supply by 1.2 million bpd from Jan. 1 to prevent excess supply building up. OPEC's share of that cut is 800,000 bpd.

In the report, OPEC said its oil output fell by 797,000 bpd month on month to 30.806 million bpd in January. That amounts to 86 percent compliance with pledged cuts, according to a Reuters calculation.

(Reporting by Alex Lawler Editing by Edmund Blair)