South African President Kgalema Motlanthe said the bloc, the Southern African Development Community, opened an investigation into the allegations when Mugabe's regime first raised them last month. But Motlanthe, the current SADC chairman, added: "We never believed that."
SADC also includes Botswana, which dismissed the allegations last month and again when Zimbabwe officials raised them again this week.
The Zimbabwe opposition also has repeatedly dismissed the allegations, calling them part of a plot to create a pretext for declaring a state of emergency that would give Mugabe broad security powers.
Also this week, Zimbabwe state media reported the head of the country's air force was wounded in what "appears to be a buildup of terror attacks targeting high profile persons, government officials, government establishments and public transportation systems."
Mugabe, in power since independence from Britain in 1980 and seen as increasingly autocratic, and the opposition have been deadlocked over a power-sharing agreement since September. The political impasse comes amid a mounting economic and humanitarian crisis that has pushed thousands of Zimbabweans to the point of starvation and left nearly 1,000 dead of cholera since August.
Motlanthe, speaking to reporters in the South African capital of Pretoria, would not say why he thought Mugabe's regime was pressing allegations the opposition was plotting violence, but noted there was "mistrust" among Zimbabwe's politicians.
In another measure of that mistrust, Motlanthe said a SADC plan to send Zimbabwe humanitarian aid hinged on the creation of a new, nonpartisan agency being established to distribute food and medicine. Motlanthe said Zimbabwe had a history of allegations of aid being hijacked by politicians and not being distributed fairly.
"It is important for the relief to reach all people of Zimbabwe without being influenced by partisan interests, political interests," Motlanthe said.
SADC has proposed an umbrella aid agency that would include all political parties, international aid agencies, Zimbabwean farmers and others, Motlanthe said.
Much of what Motlanthe said Wednesday could be read as criticism of Mugabe's leadership, but the South African stopped short of explicit denunciations. South Africa has long argued that confronting Mugabe could backfire.
Motlanthe said even though nations like Britain have called for Mugabe to step down, South Africa would be guided by Zimbabweans. Zimbabwean political leaders agreed in September that Mugabe would remain president and opposition leader Morgan Tsvangirai would be given the new post of prime minister.
"The issue of whether President Mugabe should go or not has never been raised by the parties," Motlanthe said. "We feel that we should really support and take our cue from what they want."
Motlanthe called for the unity government to be formed quickly, "because only then would we be able to deal with the real problems facing Zimbabwe." He added he believed the coalition could be formed as soon as this week, but sentiment in Zimbabwe is much less optimistic.
Africans have been under pressure to take a firmer stand against Mugabe. Tanzania's President Jakaya Kikwete, who heads the African Union, on Tuesday dismissed accusations that SADC and the AU were doing too little to resolve Zimbabwe crisis.
"We have managed to push Mugabe and Tsvangirai to sign an agreement in September for power sharing. This is good for Zimbabweans and Africa," Kikwete told reporters during a visit to neighboring Mozambique.
He said Zimbabweans should work very hard to get the agreement implemented quickly.
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