A squeeze on funding caused by the spread of crises across the Middle East and Africa has left the World Food Programme (WFP) unsure about whether it can continue its planned operations in Afghanistan, the UN food organisation said on Friday.

"We met donors and implored them to continue their support to this country to ensure we don't lose momentum," Ertharin Cousin, executive director of the WFP, told reporters during a four-day visit to Kabul.

She said the organisation, which has been active in Afghanistan since 1963, faced a $50 million funding shortfall to support its programs this year and ensure they were not interrupted during the coming winter.

A surge in the number of refugees returning from Pakistan had added to the strains, she said.

Angeline Rudakubana, the WFP's deputy country director for Afghanistan, said the organisation had been forced to reduce rations in its school meals program and cut other support.

Chronic insecurity in many parts of the country had occasionally forced the WFP to suspend some activities, but the main problem remained funding.

The comments underline the extent to which aid to Afghanistan, where the WFP estimates that 40 percent of people face "food insecurity", is under strain 15 years after the hardline Taliban regime was toppled.

"It is not donor fatigue. Globally, the donors have never been more generous," Cousin said.

"But we are seeing increased demands for donor support, whether it's Syria, Iraq, Yemen, South Sudan, now northeast Nigeria."