At a news conference, Ban also issued thinly veiled criticism of efforts by the Southern African Development Community (SADC) to mediate in the Zimbabwe crisis, saying they had failed to deliver results.
Zimbabwe is suffering from runaway hyperinflation, shortages of food and fuel, and a cholera outbreak that has killed nearly 1,000 people. A September power-sharing agreement between the ruling party and opposition has not been implemented.
The United Nations says 5.8 million people, more than half the population, will need food aid in the first quarter of next year.
"I told (Mugabe) things needed to change, urgently, and that I and the U.N. stand ready to help. The president agreed to receive my envoy, Haile Menkerios. Now we are told that the timing is not right. If this is not the time, when is?" the U.N. chief said.
Echoing remarks he made to the Security Council on Monday, Ban said the world needed a "fair and sustainable political solution in Zimbabwe … and we need it fast."
In pointed comments, he said that for the past eight months, the SADC had "insisted on leading international diplomatic efforts — with little result.
"When the international community or a regional organization takes on a mission, it also takes on the responsibility to deliver," he said.
South African President Kgalema Motlanthe said on Wednesday the SADC would "launch an urgent international campaign to mobilize financial and material resources to help the people of Zimbabwe … overcome the challenges facing their country."