Brent crude, the global oil benchmark, was down 1.6% on the day, at $73.54 a barrel, on London's Intercontinental Exchange.
Iran said on Tuesday OPEC was unlikely to reach a deal, setting the stage for a clash with kingpin Saudi Arabia and non-member Russian Federation, which are pushing to raise production steeply from July to meet growing global demand.
Opec sources also said Iran had demanded that US sanctions be mentioned in the group's post-meeting communique, as Tehran has blamed USA measures for the recent rise in oil prices. That would effectively mean a modest boost from producers such as Saudi Arabia that have voluntarily cut more deeply than planned.
"OPEC could keep the same deal with compliance going back to 100%", said an OPEC source who is aware of Iran's stance.
The uncertainty heading into today's OPEC meeting has kept numerous major hedge funds on the sidelines for weeks because even with their resources, they have been unwilling to place bets on either side of the market, given the volatility usually associated with an OPEC meeting.
The 24 nations in the supply-cat pact, known as OPEC+, agreed in late 2016 to trim production by 1.8 million barrels a day but they have actually been keeping some 2.8 million barrels per day off the market. Oil production by state-run ONGC stood at 1840.93 TMT during the month, which is 4.82 per cent lower when compared to May 2017 production.
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Saudi Energy Minister Khalid al-Falih had earlier signalled a compromise could be in the works.
Opec and its allies have since previous year been participating in a pact to cut output by 1.8 million bpd.
Mr Zanganeh has said the president is to blame for high prices because of his unilateral withdrawal from the global nuclear agreement and the imposition of fresh sanctions that could significantly curb Iran's crude exports.
OPEC is meeting in Vienna on Friday and Saturday and is expected to discuss potentially raising crude oil production now that prices have stabilized so far in 2018.
Iran's oil minister, Bijan Namdar Zanganeh, questioned this week whether OPEC is united enough to reach any agreement, and said Trump had politicized the debate over oil prices. Iran is usually not part of the committee, which includes Russia, Saudi Arabia, the United Arab Emirates, Oman, Kuwait, Algeria and Venezuela.
For about three weeks ahead of the meeting, prices had retreated from 3-1/2-year highs on fears that larger production increases could lead to oversupply.
"It would be nice if we can do it in 2019", al-Falih said at an OPEC conference in Vienna today, Bloomberg reports. The paper agreement would see production increase by 1 million barrels a day to bring the group back to target.
Russia, which isn't a member of OPEC but has gained influence in the oil group, wants an increase of 1.5 million bpd.