Opec and its allies are working towards a deal to reduce oil output by at least 1.3 million barrels per day, four sources said, adding that Russia's resistance to a significant production cut was so far the main stumbling block.
Opec meets on Thursday in Vienna, followed by talks with allies such as Russia on Friday, amid a drop in crude prices caused by global economic weakness and concerns about an oil glut due largely to a rise in US production.
The producer group's de facto leader, Saudi Arabia, has indicated a need for steep reductions in output from January but has come under pressure from US President Donald Trump to help support the world economy with lower oil prices.
Possibly complicating any Opec decision is the crisis around the killing of journalist Jamal Khashoggi at the Saudi consulate in Istanbul in October. Trump has backed Saudi Crown Prince Mohammed bin Salman despite calls from many US politicians to impose stiff sanctions on Riyadh.
The sources, three from the Organization of the Petroleum Exporting Countries and one from a non-OPEC producer, said the meeting was taking place in a difficult environment and that Russia's position would be key in reaching a deal.
"Russia is playing tough," one of the Opec sources said.
Russian sources have indicated the country could contribute some 140,000 bpd to a reduction, but Middle East-dominated Opec insists Moscow cut by 250,000-300,000 bpd.
Two Opec sources said talks were focusing on a pro-rata cut of 3-3.5 per cent from October output levels, with no exemptions for any member.
"Until those main criteria are met, Opec could endure a fall in oil prices for a while if it opted to delay a decision on a cut for a month or two," one of the sources said.
"Opec can always meet again in February and decide on a cut then. Those who were not able or willing to cooperate will be wanting to cut then," the source added.
Qatar said on Monday it would quit Opec to focus on gas in a swipe at Saudi Arabia, which is trying to show unity in tackling the slide in oil prices.