KABUL, Afghanistan -- Despite improved agricultural production, an estimated 4 million Afghans will face food shortages in the next year because of long-time drought, the U.N. World Food Program warned Sunday.

About 1.4 million of those people are living in areas that will be hard to get to because of the onset of winter in mid-November.

In order to reach many of the isolated communities, the U.N. agency will distribute 56,100 tons of wheat before winter, said spokesman Alejandro Lopez-Chicheri in a statement.

WFP has already distributed 8,800 tons of food aid from warehouses in four locations across the country, the statement said.

U.N. officials said this year's cereal harvest was 80 percent better than the previous year because of increased rainfall, but pockets of drought still remain.

"There are still large numbers of vulnerable people who have been affected by conflict and several years of drought," the statement said. "It should be emphasized that, although the overall situation is improving, there are still areas of low agricultural production due to limited or late rainfall."

The deputy secretary-general of the United Nations arrived in Afghanistan's capital on Sunday for a three-day visit, U.N. officials said.

Louise Frechette was to meet with President Hamid Karzai later Sunda.

Frechette will also hold talks with several Cabinet ministers.

Meanwhile, the U.N. children's agency said it was launching a three-day campaign Tuesday to vaccinate 5.9 million children against polio.

UNICEF denied a report that a girls' school in Narkh district in Wardak province was destroyed by armed assailants. UNICEF officials visited the school and found no evidence it was attacked.