"Even with tentative signs that bulging inventories are starting to decline, our supply-demand outlook suggests that the market — if left to its own devices — may remain in oversupply through the first half of next year," the IEA said in its monthly report.
"If Opec sticks to its new target, the market's rebalancing could come faster."
But the IEA recognised that it is currently difficult to assess exactly how the Opec supply cut, if enforced, will affect the market.
"A significant rebound in production from Libya and Nigeria and further growth from Iran would suggest that bigger cuts would have to be made by others, such as Saudi Arabia, to meet the ... production target," it said.
"The extent of any cooperation from non-Opec producers, such as Russia, is still to be determined."
The IEA expects global oil demand to rise by 1.2m barrels per day (bpd) next year, unchanged from last month. However, its growth estimate for this year was cut to 1.2m bpd from 1.3m bpd.
Opec has provisionally agreed to cut production by around 700,000 barrels per day, a move which would limit its output to between 32.5m and 33m barrels per day.
The oil cartel's members are due to meet in Vienna on 30 November to finalise the agreement, a move which would end its strategy of increasing production to defend its market share against the emergent US shale gas industry.