Khalid al-Falih said he was hopeful an agreement would be reached on Friday, minutes after meeting Iran, which has been the key holdout to a rise in output.

Delegates privy to the negotiations in Vienna told the Guardian talks had been fraught but they thought the cartel could maintain unity despite divisions.

“An agreement was reached yesterday to release the equivalent of about 1m barrels to the market; it will be distributed pro rata,” Falih said on Friday, of an Opec committee meeting on Thursday. The deal has not been formally signed off yet, however.

“Saudi Arabia is unique. All of our spare capacity is available at short notice,” he said. He acknowledged not all of the cartel’s members could increase output, in a reference to countries such as Venezuela.

Falih said the world should not expect the impact to be felt until the end of summer because crude would take weeks to reach markets.

Opec and Russia have come under pressure from the US, China and India to boost production, with India’s oil minister in Vienna this week complaining about the impact of prices on consumers.

Iraq’s energy minister said it was clear that Opec and its allies would change course after 18 months of production curbs, in an effort to rebalance crude supply and demand.

Asked if there would be an increase in supplies, Jabbar Alluaibi said: “Definitely a change.”

After nearly two years of relative unity, the talks in Vienna this week have found Opec divided. After arriving on Tuesday, the Iranian oil minister, Bijan Zanganeh, threatened to veto a Saudi and Russian-led drive to raise output, and observers said “the body language has been awful”.

The Ecuador oil minister, Carlos Pérez, when asked if the talks had been fractious, said: “It’s been a difficult situation.” However, he added the cartel hoped to continue its unity despite the disagreements.

Iran is understood to want a coded criticism of the US, which recently ended an international nuclear agreement with Iran, in the final Opec agreement on Friday.

Opec used the opening of the meeting to make it clear the group did not want to see prices sliding too much because of a rise in supplies. 

“We need to continue to tread carefully; none of us want to see the return of the kind of volatility that allows pessimism to return to the markets,” Suhail Mohamed Al Mazroui, the UAE energy minister and president of the Opec conference, said.

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