The agency said it was working with Mr Mugabe's government and other international aid organisations to provide food assistance to about one-fifth of Zimbabwe's 13 million people from next month until the crop harvest next March or April.
The UN Food Programme said in a statement released yesterday: "Hunger is on the rise in Zimbabwe with an estimated 2.2 million people - one in four of the rural population - expected to need food assistance during the pre-harvest period early next year."
That is the highest number of Zimbabweans requiring food assistance since early 2009, when more than half the population relied on such aid.
That was the peak of a decade-long economic crisis critics blame on Mr Mugabe's policies, notably his government's seizure of white-owned commercial farms for redistribution to landless blacks.
Mr Mugabe, 89, who has been Zimbabwe's ruler since independence from Britain in 1980, maintains he was correcting imbalances in ownership which had been created by colonialism.
The latest food shortages were due to bad weather, high seed and fertiliser costs and projections that food prices will climb because of the poor maize harvest.
Mr Mugabe and his Zanu-PF party, who were declared overwhelming winners in a July 31 election rejected as a fraud by his main rival Morgan Tsvangirai, have promised food imports and said no Zimbabwean would die from hunger.
Mr Tsvangirai had been involved in power sharing with Mr Mugabe, but that ended after the election.