LONDON - OPEC oil output has fallen in April, a Reuters survey found on Tuesday, reflecting lower exports from Iran, Iraq and Nigeria against a backdrop of ongoing voluntary supply cuts by some members agreed with the wider OPEC+ alliance.

The Organization of the Petroleum Exporting Countries pumped 26.49 million barrels per day (bpd) this month, down 100,000 bpd from March's revised total, the survey, based on shipping data and information from industry sources, found.

Several members of OPEC+, which includes OPEC, Russia and other allies, made new cuts in January to counter economic weakness and increased supply outside the group. Producers agreed in March to keep the cuts in place until the end of June.

OPEC+ is scheduled to meet on June 1 to decide its next output policy steps.

The biggest output reductions in April came from Iran, Nigeria and Iraq, the survey found.

Output in Iran, exempt from quotas, slipped from an upwardly revised March figure that equalled a five-year high reached in November, the survey found. Iran posted one of OPEC's biggest output increases in 2023 despite U.S. sanctions still in place.

Iraq in March promised to lower exports to make up for pumping above its OPEC target, but data on Monday showed it missed the pledge last month. April output is down slightly from March, according to the survey.

Nigerian production declined, with exports falling more sharply according to some ship trackers as the Dangote refinery took in more cargoes. An outage briefly affected the Bonny production stream, a source said.

OPEC fell about 140,000 bpd short of its targeted cuts in April, largely because of Iraq and Gabon pumping more than they had aimed for, the survey found. Nigeria is pumping below its target, unlike earlier in the year.

Saudi production edged up and there were small increases from Congo, Equatorial Guinea and Gabon, the survey found.

The Reuters survey aims to track supply to the market and is based on shipping data provided by external sources, LSEG flows data, information from companies that track flows - such as Petro-Logistics and Kpler - and information provided by sources at oil companies, OPEC and consultants.

(Additional reporting by Ahmad Ghaddar Editing by Tomasz Janowski and David Gregorio)