South Korea has taken a tougher line on relations with the North since the election of conservative Lee Myung-bak as president late last year.
Although the government has said it will not tie aid to progress on North Korea's nuclear disarmament, it has also said public opinion in the South would play a consideration in deciding whether to send aid.
Public sentiment in the South has worsened considerably following the July shooting death of a South Korean tourist at a North Korean mountain resort.
Up to two million North Koreans are thought to have died in a famine in the late 1990s after a combination of mismanagement and natural disasters devastated the country's centrally-planned economy.
In its report the WFP said higher food prices, reductions in public food rations and lower employment had made the impact of the shortages particularly severe for urban households in areas with low industrial activity.