UN staff in Herat say their warehouses are secure, but almost empty.
The WFP is urgently trying to organise food convoys from Iran and is confident that Iran and the Taleban will allow the lorries through.
"If we had trucks, we have 3000 tonnes of American food ready to go, and 60,000 tonnes of aid easily available in the region," said Mr de Gaay Fortman.
"The main hiccup is to find Afghan drivers prepared to take it."
Food would be unloaded at the Iranian border and then put on Afghan transport.
"My people in the field tell me Afghan drivers are hiding their vehicles in safe places and are not so interested in business.
"I think they are scared their trucks will be damaged.
"My experience is that, even in a war area, you still see trucks going backwards and forwards. But usually you are dealing with two factions fighting on the ground and the drivers can buy their way through."
- AGENCIES