A senior UN official said the intensity of the crisis was increasing faster than expected and said the situation had deteriorated most in Zambia and Zimbabwe, where a disaster could only be averted by urgent action.
World Food Programme director James Morris said drought had combined with the HIV/Aids pandemic and politics to create "a crisis of incredible proportions".
His estimate, that 14.4 million people need food aid, represents a 1.6 million increase on the previous UN figure and constitutes the region's worst food crisis in a decade.
"The needs are immense and immediate. Failure to address all of the needs now will only extend the crisis," he said in a statement at the end of a two-week tour of the region.
Just over one-third of the $500m requested by the WFP had been confirmed so far, he said..
GM progress
His latest assessment of the crisis affecting Zimbabwe, Zambia, Malawi, Mozambique, Angola, Swaziland and Lesotho was conducted as a new planting season begins.
"Prospects for next year's harvest are bleak unless small-scale farmers immediately receive adequate supplies of seeds and fertiliser in time for the planting season, just one month away," he said.
In Zimbabwe, the government sponsored seizures of white-owned farms are being blamed for worsening the food shortages.
Mr Morris has held a series of talks with President Robert Mugabe, during which he said he had made "incremental progress" on food security issues.
The safety of genetically-modified (GM) food aid has also emerged as a complicating factor, though the UN feels it is making progress is in allaying the concerns of Southern African nations.
Currently, only Zambia is refusing to accept GM grain from the United State.
Mr Morris said a team of Zambian scientists is now assessing its safety.