UN chief Ban Ki-moon says Zimbabwe cannot afford to fail in negotiating a power-sharing deal if the country is to improve its humanitarian situation.
Mr Ban said President Robert Mugabe's Zanu-PF and the opposition MDC needed a workable agreement soon, so they could tackle "formidable challenges" ahead.
Representatives of the parties are said to have resumed talks in South Africa.
Mr Ban also said he was concerned by a cholera outbreak in Zimbabwe, which the UN says caused 53 deaths on Monday.
The UN Office for the Co-ordination of Humanitarian Affairs said the fatalities brought the total since August to 366. The number of recorded cases increased by 1,604 in the past day to 8,887.
The agency said most of the deaths were reported in the town of Beitbridge, which is located close to the border with South Africa.
It said the news, along with reports of several suspected cases in Botswana, meant the outbreak was taking on a "regional dimension".
Earlier, the mayor of the nearby South African town of Musina expressed fears about a possible cholera epidemic as infected refugees arrived from Zimbabwe.
Mr Ban also said he was deeply concerned that nearly half of the country's population of 12 million people could require food assistance.
He added that he was distressed by the "collapse of health, sanitation and education services, and the consequent rapidly escalating cholera outbreak".
On Tuesday, the Sapa news agency reported representatives of the Zanu-PF and the Movement for Democratic Change (MDC) resumed negotiations on a national unity government at an undisclosed location in South Africa.
Former South African leader Thabo Mbeki is hosting the talks, during which the two rivals will attempt to agree on the distribution of key ministries.
South Africa has recently increased pressure on both sides to reach an agreement. Last week, it said it would withhold $US28 million of aid until a representative government was formed.