Africa is in need of a "green revolution" to combat a growing food crisis on the continent, former United Nations secretary-general Kofi Annan said in a speech in London on Wednesday.

Speaking at King's College London, the Ghanaian diplomat also said that more needed to be done to deal with the impact climate change would have on food supplies in Africa, and added that immediate action was necessary to stave off thousands of deaths.

"The most pressing challenge we face is food supply," the 70-year-old Nobel Peace Prize winner said.

"We need action to tackle the immediate food crisis in Africa and the long-term impact that climate change will have on food supplies and agriculture across the continent.

"Without immediate action we are certain to see, indeed are seeing, many thousands of more deaths."

According to Annan, Africa needed "nothing less than a green revolution transforming every aspect of farming on the continent," with farmers in particular needing better seeds, soils and fertilisers, as well as more support from their goverments and the international community.

World Bank figures show that global food prices have nearly doubled in three years, with experts blaming rising oil prices and the growing use of biofuels, among other factors.

Annan also addressed the ongoing political crisis in Zimbabwe, which he described as "both intolerable and unsustainable" and as "tarnishing the reputation of Africa."

"Regardless of who wins the election you need to work with the two parties to determine how they manage the country -- regardless of who wins," he said.

Zimbabwean President Robert Mugabe and opposition leader Morgan Tsvangirai will participate in a run-off vote next month, with the country in economic freefall -- the official rate of inflation stands at over 165,000 percent, and unemployment is around 80 percent.