The WFP and other UN agencies fear that up to 1.5 million Afghans could flee into Pakistan if the United States carries out military strikes in Afghanistan.
Click here for map of refugee movements in Afghanistan
Warehouses in and around Peshawar have been stocked with tens of thousands of blankets, plastic sheets, tents and other items.
Food stockpiled
Large amounts of food have been stockpiled and more is on the way.
UN officials said the United States had promised to provide enough wheat to feed a million people for a year.
But it is still not clear where the refugees will be sheltered.
Some are already arriving in the Pakistani cities of Quetta and Peshawar.
Pakistan's refugee crisis
But the Pakistani authorities said they wanted to keep the new refugees separate from Afghan refugees already living in Pakistan.
UN experts are now surveying about 100 possible sites for new camps near the border.
But it will be at least a week before any of the sites will be ready to receive refugees.
Pakistan and Iran are already sheltering 3.5 million Afghan refugees - the largest refugee group in the world.
UN appeal
The UN Secretary-General, Kofi Annan, has appealed for $584m to help millions of Afghans at risk of starvation in Afghanistan and bordering countries.
Eight million people are expected to need food aid inside Afghanistan this winter.
The UN says it is the world's worst humanitarian crisis.
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