The United Nations food agency has negotiated a humanitarian corridor to keep food aid flowing in southern Africa after most countries shut their borders to stop the spread of coronavirus, an official said on Thursday.

Up to 45 million people in southern Africa face hunger following a devastating drought and two cyclones last year, and there are growing fears the situation could be compounded by the outbreak of coronavirus.

Lola Castro, World Food Programme director for southern Africa, said South Africa had agreed to let vessels carrying food aid land on its shores and move it to countries like Zimbabwe, Malawi, Botswana and Namibia, which face food shortages.

Castro told reporters during a video conference that Africa's most advanced economy, which has declared a 21-day lockdown, had agreed to "a sort of a humanitarian corridor", a move that would help the agency continue its work.

"At this moment we are actually maintaining our normal food distributions but maybe in the future, depending with the effect of the virus and effect on the food system, especially on the smallholder farmers ... maybe we will see an increase in prices and number of people who require food assistance," she said.

More than $400 million was required to import food aid for the next three months in southern Africa, which has received patchy rains this year, said Castro.

Africa has now registered almost 6,445 cases of coronavirus and 241 deaths. The continent is already suffering a huge economic impact from lockdowns aiming to contain the virus and a sharp fall in global demand for commodities. Some international aid workers have been prevented from doing their jobs by closed borders.

Daily lives have been upended in most African countries due to tough lockdown measures that have affected swathes of populations that rely on informal trade for a living.

Matshidiso Moeti, World Health Organisation regional director for Africa, said governments on the continent had to balance between physical distancing measures while making sure that families had food on the table.

"The potential economic impact is of great concern at the lowest socio-economic levels," Moeti said.

Authorities in Zambia said the country had recorded its first death from coronavirus, and the number of confirmed cases had risen by three to 39.