In its monthly Oil Market Report the IEA said Opec crude production rose further above quota limits in December to stand 4.6 million barrels a day above projected demand for the cartel's crude in the second quarter.
Supply usually outstrips demand in the second quarter when demand eases after the northern hemisphere winter, allowing inventories to rebuild.
But some Opec ministers are worried this year's second quarter surplus will be much larger than normal, causing a price crash. Friday's IEA report was the last before Opec meets on February 10. The IEA said December Opec production rose to 27.95 million bpd, compared to its 23.3 million bpd forecast for the 'call', or demand for, Opec oil in the second quarter.
Assuming Opec output at December levels, the projected 4.6 million bpd stockbuild is 3.1 million higher than the 1.5 million bpd build considered by Opec's chief economist as normal for the second quarter.
Some analysts say that while stocks need to rise to replenish US crude oil inventories, now at their lowest level since 1975, a build of this magnitude would be too large for Opec's comfort.