Carter and Annan, clearly exasperated after Mugabe's government blocked their scheduled visit to Harare, denying them visas on the grounds their visit was unnecessary, called on regional leaders to help enforce the implementation of the unity government.
Annan, flanked by Carter, and human rights campaigner Graca Machel, the wife of Nelson Mandela, said on Monday: "Sadc [the Southern African Development Community leaders group] must bring its full weight to bear... I think it is clear that Sadc should have done more."
Machel, who, along with Carter and Annan, is part of a group called the Elders, lauded South Africa's move to withold $28 million in food aid from Zimbabwe in an attempt to force the leadership to share power.
"That's a good tone. Maybe other Sadc nations should consider doing the same," she said.
Talks led by Thabo Mbeki, former South African president, are expected to resume on Tuesday in South Africa.
South Africa's ruling African National Congress (ANC), backed by Mugabe during its decades-long struggle to overthrow apartheid, will also send a delegation to Zimbabwe to assess the situation, said ANC leader Jacob Zuma.
Zuma also voiced his alarm at the rapidly-deteriorating humanitarian situation in Zimbabwe.
"The situation has just gone beyond a situation where we could say 'wait and see'," he said.