Output from the Organisation of the Petroleum Exporting Countries (Opec) grew last month due to ramped up production in Nigeria and Libya
Combined, the two countries' production increased by more than 350,000 barrels per day (bpd) in May, pushing Opec's crude oil production to an average of 32.14m bpd, according to Opec's monthly oil market report.
Total production was 31.8m bpd in April.
Libya and Nigeria are exempt from the cartel's landmark supply cut agreement that began in January. Under the agreement, Opec and other nations, including Russia, pledge to cut production by 1.8m bpd in the first half of the year.
Last month, the deal was extended for a further nine months as Opec works to rebalance the market.
However, Opec's cuts have been undercut by huge a increase to US shale drilling activity, which is causing the country's output to rise.
Opec said the rebalancing of the oil market is under way at a "slower pace" due to "the shift in US supply from an expected contraction to positive growth".
Oil prices were trading lower this afternoon. The global benchmark Brent crude was 0.25 per cent lower at $48.17 a barrel while West Texas Intermediate (WTI) was 0.3 per cent lower at $45.94 a barrel. Crude prices have been slashed by 10 per cent since Opec announced its extend production cuts last month.