Some traders said crude prices were partly pressured by the dollar's recovery following comments by European Central Bank President Mario Draghi that were considered bearish for the euro.
Despite rising output by OPEC's Middle Eastern producers, the group's overall production has remained largely flat this year, currently standing at 32.5 million barrels per day (bpd), capped by disruptions especially in Nigeria, Libya and Venezuela.
Citi said it expected oil prices to rise above $50 per barrel "in the near future" as attacks on oil infrastructure in Nigeria, power outages and payment issues in Venezuela and chaos in Libya have reined in total OPEC production even as Iran has ramped up harder and faster than expected.
Because of supply disruptions elsewhere, the Middle East's low cost producers see little reason to restrain output as overall market conditions have improved significantly for them this year.
"Since January 20, oil prices have almost doubled from near $26 per barrel to almost $50 per barrel, at the same time total OPEC production has increased by around 100,000 bpd, despite heavy outages in Nigeria," BMI Research said.
"This indicates the strategy championed by Saudi Arabia... to let the oil market balance without intervening, is gradually playing out," it added.