Oil inventories may accumulate by 700,000 barrels a day in the first quarter even if the Organization of Petroleum Exporting Countries and its allies implement the entire cutback of 2.1 million barrels a day agreed last week, the IEA said in a monthly report. Supplies outside the group, led by U.S. shale, continue to grow much faster than world demand.

While crude prices climbed to a 12-week high in New York after OPEC+ surprised traders with their latest intervention, they remain below $60 a barrel amid concern that the additional output curbs still won’t be enough. The U.S. briefly became a net exporter of oil three months ago, underscoring the challenge OPEC faces from America’s shale boom.

“The market has done its own sums and the reaction to oil’s new deal has so far been muted,” said the Paris-based agency, which advises most of the world’s major economies.

The extra OPEC+ curbs would translate into an actual reduction from current levels of 532,000 barrels a day, the IEA said. Implementing that fully may be a tall order, as some producers like Iraq and Nigeria have barely made the cutbacks they promised to enact this year.

A change to the terms of the OPEC+ agreement, which now exempts light oil known as condensate produced by the non-OPEC members, could also undermine the coalition’s efforts. Condensate production in those countries, such as Russia and Azerbaijan, has the potential to increased from its current level of about 1.5 million barrels a day, according to the IEA.

Saudi Arabia, OPEC’s biggest member, has already made considerable progress in fulfilling its output commitments, including additional voluntary reductions announced at the close of the OPEC meeting on Dec. 6. The kingdom pumped 9.9 million barrels a day in November, the IEA estimated.

“The overall effectiveness of the OPEC+ agreement depends on the willingness of all its parties to fully comply, including those whose compliance so far has been less rigorous,” the agency said.

OPEC and its partners are getting some solace from a recent recovery in oil demand. Global consumption increased at the strongest rate in a year during the third quarter, expanding by 900,000 barrels a day, according to the report. That’s almost twice the pace seen in the second quarter.

Demand is set to accelerate further in 2020, when it will expand by 1.2 million barrels a day — about 1.2% — to average 101.5 million barrels a day, the IEA predicts.

OPEC may also be reassured that supply from some of its rivals, such as the U.S., Brazil and Ghana, is increasing less quickly than the IEA previously forecast. The agency lowered its projections for non-OPEC production growth in 2020 by about 200,000 barrels a day.

Yet supplies outside OPEC will nonetheless expand much more vigorously than world demand, swelling by 2.1 million barrels a day next year as a new tide of American shale-oil is joined by offshore projects once considered unviable in an era of constrained prices, from Brazil, Norway and Guyana.

As a result, the surplus in global markets may swell to as much as 1 million barrels a day during the second quarter, the agency said. That poses a considerable challenge for the cartel and its allies, who will meet again in early March to consider their next move.