The latest round of cuts were due to expire Sunday.
Member representatives are meeting Monday at OPEC headquarters in Vienna.
Khalid Al-Falih, Minister of Energy, Industry and Mineral Resources of Saudi Arabia arrives for a meeting of the Organization of the Petroleum Exporting Countries, OPEC, and non OPEC members at their headquarters in Vienna, Austria, Monday, July 1, 2019.
Financial markets, meanwhile, were buoyed by a thawing of U.S.
Russian Energy Minister Alexander Novak said that Russian oil output last month fell by 278,000 barrels per day (bpd) from an October 2018 baseline of 11.41 mln bpd, Russian news agencies reported, indicating output in June of about 11.13 million bpd.
"The reason for extending the deal by nine months instead of six is to assure the markets that the deal will remain in place through the seasonally soft demand period in the first quarter of 2020", said Amrita Sen, co-founder of Energy Aspects.
Analysts believe that the extension of the cuts for nine months is going to be cheered on by the market.
Head of the "Caspian Barrel" Center for Oil Research Ilham Shaban told Vestnik Kavkaza that the global oil market has only positive expectations from this summit. "They will only have to pay a small restraint while reaping a nice oil price of $60-70 a barrel", said SEB's Bjarne Schieldrop.
"They (OPEC members) aren't going to agree to anything that Russian Federation hasn't already signed on to". Tehran's line is clear. Iran has denounced the sanctions as illegal.
The expert pointed out that the deal has allowed to keep oil prices in the same price corridor for several years.
"An organization, where two members strive to challenge the interests of other members, is doomed to dissolution and talks of OPEC-non-OPEC agreements would be meaningless", Zanganeh said.
Front-month Brent crude futures, for September, touched an intraday high of $66.63 a barrel and were up $1.72, or 2.7%, at $66.46 a barrel by 0639 GMT.
A senior analyst of "Uralsib", Alexei Kokin, does not expect any surprises from today's negotiations.
The main intrigue of the 1-2 July meetings has been somewhat subsided by Russian Federation and Saudi Arabia's announcement after the G20 meeting in Osaka about their aligned position to extend the cuts with current production levels by six-nine months.
A 9-month extension would mean the deal runs out in March 2020.
Iranian officials, however, have dismissed such moves as psychological warfare, saying the country has its own ways of circumventing the American bans and selling crude oil.
The current deal removed 1.2 million barrels a day of OPEC+ production, helping to send oil prices higher.
"It's not only my mission, it's a matter of Opec as well because this unilateralism threatens Opec as a whole, not only Iran".