The World Food Programme (WFP) said on Wednesday that it needed $220 million in funding to provide assistance to drought-hit Zimbabwe until March 2017.
The number of Zimbabweans requiring food aid has risen to 4 million, up from 3 million initially, as the southern African nation grapples with its worst drought in more than two decades.
Zimbabwe faces its worst malnutrition crisis in 15 years, with tens of thousands of children requiring urgent treatment, UNICEF said.
The number of hungry families doubled in the past eight months "as nearly 33,000 children are in urgent need of treatment for severe acute malnutrition," UNICEF representative, Jane Muita, said in a statement.
"We have not seen these levels of malnutrition in more than 15 years and... more needs to be done to prevent this crisis from spiralling out of control."
"Severe acute malnutrition" is defined as extreme hunger causing visible wasting and fluid retention.
"Water scarcity is also exposing children to higher risks of diarrhoea, typhoid and other waterborne diseases including cholera," Muita added.
UNICEF is requesting $21 million in aid to meet the needs of children in Zimbabwe.
Some families are saving dwindling stocks by skipping meals, while schoolchildren are missing classes due to hunger, according to local media reports.
In addition to the effects of severe drought, Zimbabwe has suffered perennial grain shortages after land reforms under which commercial farms owned by whites were redistributed to landless blacks.
The state-owned Herald newspaper on March 15 quoted social welfare minister Prisca Mupfumira as saying up to four million people required food assistance nationwide.