More than half the population - about 22.8 million people - face acute food insecurity, while 3.2 million children under five could suffer acute malnutrition, the WFP said.
"Afghanistan is now among the world's worst humanitarian crises, if not the worst," said David Beasley, the executive director of the WFP.
"We are on a countdown to catastrophe."
Afghanistan fell to the Taliban in August after the US pulled out the last of its remaining troops and the militants swept across the country retaking ground.
The takeover weakened an already fragile economy that was heavily dependent on foreign aid. Western powers suspended aid and the World Bank and International Monetary Fund also halted payments.
A nation is considered aid-dependent when 10% or more of its gross domestic product comes from foreign aid; in Afghanistan's case, about 40% of GDP was international aid, according to the World Bank.