Africa's heads of government have called on Zimbabwe's political parties to initiate a dialogue aimed at setting up a government of national unity.

They have also pressured the Southern African Development Community to "seize the momentum for a negotiated solution" to the country's political crisis by establishing a mechanism "on the ground" in Zimbabwe.

A summit of the African Union adopted a resolution on Tuesday night which noted that three groups of observers - from SADC, the Pan-African Parliament and the AU itself - had issued "negative reports" on last Friday's presidential run-off vote in Zimbabwe.

The summit said there was an urgent need "to prevent further worsening of the situation" and to avoid the spread of conflict. There was also a need to "to create an environment conducive for democracy" in Zimbabwe.

It said preparatory talks on a unity government had already started, and encouraged President Robert Mugabe and Morgan Tsvangirai, leader of the Movement for Democratic Change, "to honour their commitment to initiate dialogue." The summit went on to give explicit support for a government of national unity.

News agency reports from the summit, held in the Red Sea resort of Sharm el-Sheikh in Egypt, said the resolution was adopted by Africa's leaders after two hours of debate on Tuesday evening. The text of the resolution was released by the South African government early on Wednesday.

Reuters interpreted the resolution as "a rare AU intervention in an internal political dispute and an unprecedented rebuff to Mugabe." But the agency said the resolution was not as strongly-worded as some nations had wanted.

Botswana reportedly wanted Mugabe banned from African Union and Southern African Development Community meetings. Vice President Mompati Merafhe said allowing him to take part gave "unqualified legitimacy to a process which cannot be considered legitimate."