Opec crude supply climbed to a two-year high ahead of a meeting of the organisations 12 member states in Vienna on 5 June, according to a survey by Reuters.
The organisation supplied an estimated 31.22 million barrels a day (b/d) in May compared with 31.16 million b/d in April, as increasing Angolan exports and close-to-record Saudi and Iraq output offset outages in smaller producers.
Opec is not expected to shift its policy on exports. The groups decision to maintain high exports was seen as a key factor in the sharp drop in crude prices in the second half of 2014.
However, the Brent crude price has increased to about $65 a barrel from a low of close to $45 a barrel in January and there are signs the rapid rise in US crude production has slowed.
Saudi Arabia, the most influential member of Opec, last year took the decision to sustain exports to maintain its market share in Asia despite the falling prices.