Emerging discord in Opec over the political wisdom of supporting high oil prices will sharpen scrutiny on whether the cartel's internal supply discipline is starting to crack.
Opec steamed ahead with a deal to reduce production quotas by four percent, despite reservations from Gulf members Kuwait and the United Arab Emirates who came under pressure from the United States to help ease fuel prices.
Analysts said the internal rift highlights a number of signals that might suggest a cartel that is overstretching itself: poor compliance with supply limits, prices far above its target band and persistent calls from several members for a bigger share of output.
The test now for Opec, which for months has revelled in the win-win of high production and high prices, will be to enforce enough of the cuts to maintain the reputation for internal discipline that has underpinned a five-year price boom.
Failure to curb supply could accelerate a price slide, sparked already by a recovery in US crude inventories, and thwart Opec's plans to manage a soft price landing. US oil on Thursday traded at $35.34, down 42 cents.
"Opec did manage to agree to a production cut despite wide-ranging views in the cartel. That looks enough to shore up oil prices. However, demand is seasonally weak, and compliance to quotas is poor," said Adam Sieminski of Deutsche Bank.
"Opec's decision... must now be followed by a firm commitment to cut output. We question whether Opec will deliver," said Doug Leggate, analyst with Citigroup bank.
A united front has been a cornerstone of Opec's market management success since 1998's price crash spurred the group's biggest producers Saudi Arabia, Venezuela and Iran to settle damaging rows.
That cohesion looks under threat after Kuwait and the UAE broke ranks and expressed unease with supply restrictions that last month helped propel US crude above $38 for its highest closing price in 13 years.
The US government said it had asked leaders of both Gulf Arab countries not to cut supply, concerned about the political fallout of rising prices at the pump during an election year.
Washington's focus on Kuwait and the UAE as principal Opec allies follows a chill in US-Saudi relations which has undermined Washington's influence over Opec's dominant producer.
Saudi Oil Minister Ali al-Naimi was influential among a large majority of Opec ministers who wanted to press on with the quota cuts that aim to prevent inventories building too quickly when fuel demand falls after the northern winter.
For now Opec discord does not run deep, and Saudi resolve to keep oil markets robust and inventories lean still has the confidence of big-money fund managers who have bet heavily on long-term price strength.
The Organisation of the Petroleum Exporting Countries has often demonstrated that downward pressure on prices quickly concentrates minds and instils supply discipline.
Saudi may have to take more of the strain than it wishes. "If you look historically at how Opec implements its cuts, Saudi Arabia will not shoulder more than half the burden," said Roger Diwan of PFC Energy.
The 10 Opec members with quotas, excluding Iraq, have been pumping about 1.2 million barrels per day above their 24.5 million bpd quota ceiling - and some 2.2 million above the 23.5 million limit that came into force on Thursday.
So far rocketing Chinese demand is helping to absorb the crude excess without any trouble, while tight supplies of US gasoline have buoyed prices across the international oil market.
Opec cannot depend on this for ever, analysts said. "Even perfect compliance with this cut will still result in an inventory build during the second quarter. At the same time, rising exports from Russia alongside the ongoing recovery in Iraqi production may partly offset Opec's best efforts," said Leggate.