IMF managing director Kristalina Georgieva said the continuation of the war meant the global lender's "reference scenario" assuming a short-lived conflict - which forecast a minor growth slowdown to 3.1 per cent and a minor increase in prices to 4.4 per cent - was no longer possible.
"This scenario, with every day that passes, is further and further behind in the rear-view mirror," Georgieva said.
The continuation of the war, a forecast of an oil price around or above $US100 per barrel, and rising inflationary pressures meant the IMF's "adverse scenario" was already in effect, she said.
Long-term inflation expectations remained anchored and financial conditions were not tightening, but that could change if the war continued, she told a conference hosted by the Milken Institute on Monday.
"Now, if this continues into 2027 and we have oil prices of $US125 more or less, then we have to expect a much worse outcome," she said.
"Then we are going to see inflation climbing up and then inevitably, inflation expectations would start de-anchoring."
The IMF in April issued three scenarios for the global GDP growth path in 2026 and 2027 amid massive uncertainty over the war in the Middle East - the main "reference forecast", a middle "adverse scenario" and the much worse "severe scenario".
The adverse scenario forecast global growth slowing to 2.5 per cent in 2026 and headline inflation of 5.4 per cent. The severe scenario forecast growth of just two per cent and headline inflation of 5.8 per cent.
Chevron chairman and chief executive Mike Wirth, speaking on the same panel, said physical shortages in oil supply would begin appearing around the world because of the closure of the Strait of Hormuz, through which 20 per cent of global crude supply passed before the war.
Economies will begin shrinking, first in Asia, as demand adjusts to meet supply while the strait remains closed because of the US-Israeli war with Iran, Wirth said.
Georgieva said the IMF was carefully tracking the slow-moving impact of the conflict on supply chains, with fertiliser already 30 per cent to 40 per cent more expensive, which would drive food prices up between three and six per cent. Other industries could also be affected.
"What I want to stress is that is really serious," she said, expressing concern that many policymakers were still acting as if the crisis would end in a couple of months and were putting in place measures to cut the impact on consumers and business, which was keeping demand for oil high.
"Don't throw gasoline on fire," she said. "Everybody in this room knows that if your supply shrinks, your demand has to follow."
