Opec in theory needs the agreement of all members to clinch a deal but has in the past agreed production pacts without Iran, which has criticised the idea of raising supply as it faces export-crippling U.S. sanctions.
Benchmark Brent crude jumped $2.19 a barrel, or nearly 3 percent, to a high of $75.24 before slipping to around $75 by 1305 GMT.
Opec has reached an agreement in principle to boost oil production, achieving a last-minute compromise that overcame Iran's threats to veto any supply hike.
"Even with this increase, we see the market remaining in deficit in H2 2018, which should see prices remain well supported", ANZ said.
Iran has so far been the main barrier to a deal, with Oil Minister Bijan Zanganeh saying on Tuesday Opec should not yield to pressure from US President Donald Trump to raise output.
Zanganeh said he would leave Vienna on Friday even before Opec holds talks with non-Opec producers the next day, but efforts were underway on Wednesday to convince Iran to participate in a deal.
"Reallocating (output) to other countries like Saudi Arabia may be a technical solution but it may not be politically acceptable to others", he said. "We have a valid deal", Iran's Opec governor Hossein Kazempour Ardebili said, responding to a question whether the agreement would be adjusted on Friday.
Russian Federation and Saudi Arabia agreed to increase outflow by around 1.5 million barrels a day to steady the market as U.S. sanctions on Iran and political instability in Venezuela are expected to affect global production.More news: Trump, GOP to huddle as outrage builds over border policy
Once known for rampant cheating on agreed oil output targets, Opec has been over-delivering on a pact in place since January 2017, cutting around one and a half times the promised amount.
Ministers have gathered in the Austrian capital to discuss a supply increase that would be equivalent to about 600,000 barrels a day, or 0.5 per cent of global supply.
"I don't think we can reach agreement" on the proposal, Zanganeh said as he left the meeting.
"While we are not in favour of prices as low as $30 per barrel, we also do not support the prevailing high price which dent our fiscal balance and undermine our development process", Mr. Pradhan added.
Saudi Oil Minister Khalid al-Falih said that an OPEC committee ahead of the official meeting had agreed "to recommend releasing the equivalent of 1 million barrels or thereabout to the market".
More important to Saudi Arabia than extra petrodollars is containing Iran, its arch-nemesis.
But the proposal has run into resistance from Iran, Iraq and Venezuela, who would struggle to immediately raise output and fear losing market share and revenues if other countries open the spigots. "OPEC is not part of the Department of Energy of the United States". He said OPEC could meet again in September to adjust the deal. He said the meeting would tackle over-compliance with oil output cuts and that Iran was "very cooperative". OPEC, he said, "is not an organization to receive the instruction from President Trump and follow it".