Ms. Machel said both South Africa’s president, Kgalema Motlanthe, and his recently ousted predecessor, Thabo Mbeki, who is mediating the Zimbabwe crisis, had sought permission from Zimbabwean authorities for them to enter the country. Mr. Carter said Mr. Mugabe himself said no.

“It seems obvious to me that leaders of the government are immune to reaching out for help for their own people,” Mr. Carter said at a news conference in Johannesburg.

Mr. Carter said Zimbabwe’s envoy in Washington had advised him that he would not be issued a visa after he applied for one several weeks ago, but Mr. Carter said the staff of the group sponsoring the trip, the Elders, thought visas would be issued at the airport. South African officials advised the humanitarian mission’s members on Friday evening that they would not be allowed to enter the country.

Zimbabwe’s foreign minister, Simbarashe Mumbengegwi, was quoted in the state-owned newspaper, The Sunday Mail, saying authorities had not barred the threesome, but had only postponed the visit because Mr. Annan had not consulted with the government on the timing and program for the visit. He said it would have been difficult for the team to meaningfully assess the situation on the ground given that Zimbabwe’s government itself had already conducted a humanitarian audit with United Nations officials.

“It is most unfortunate that the former secretary general has, for reasons best known to himself, misrepresented the position of the government of Zimbabwe,” Mr. Mumbengegwi said. “The allegations are not supported by facts.”