Traders said that a rise in output from Russia's biggest oil producer, Rosneft, also weighed on crude.
That includes adopting half-measures, such as extending the deal by just a few months or delaying a decision until next year, according to Morse.
On the supply side, tensions in the Middle East raised the prospect of disruptions, traders said.
The excess of industrialized countries' oil stockpiles over their five-year average "has fallen by more than 50% in 2017, with inventories now at around 160 million barrels", consultancy Timera Energy said. According to data, the United States bought just 525,000 barrels a day of Saudi crude in October, the lowest since May 1987 and down from 1.5 million barrels a day a decade ago.
OPEC has sought to push stocks to the five-year average.
Russia, Saudi Arabia and Iraq have already signaled they would be open to extending the curbs.
Brent crude futures were last down 34 cents on the day at $62.82 a barrel at 1212 GMT, while U.S. West Texas Intermediate (WTI) futures fell 27 cents to $56.49.
"We are seeing clear indications that the market is re-balancing at an accelerating pace and stability is steadily returning", Barkindo said.
The UAE's Mazrouei affirmed that view, saying that coordinated cuts by Opec and suppliers outside the group have helped trim crude inventories from record storage levels.
"It would have been hard to try to rebalance the market alone and so I think there is a rationale for this group to stay together... and maybe even expand", the UAE's minister said at the Abu Dhabi International Petroleum Exhibition Conference (ADIPEC), as quoted by CNBC. USA shale output, a contributor to the global glut, has responded more slowly than expected to the recent increase in crude prices, he later told reporters.
"There is definitely a willingness and a wish to expand OPEC", Al Mazrouei said, when asked if non-OPEC oil producers that are cutting production in coordination with the 14-member cartel could join OPEC eventually.
This is coming as OPEC in its November oil market report released yesterday, increased the forecast for 2018 demand for its crude by 360,000 barrels per day, from last month's report to 33.42 million bpd.