Zimbabwe's Movement for Democratic Change (MDC) has agreed to join a unity government led by Mugabe, putting into effect a deal under which, Morgan Tsvangirai, the MDC leader, would become prime minister.
Regional politicians, who have been brokering a compromise between Zimbabwean rivals, are pressing for the unity government to be formed by February 13.
Sheikh Sharif Sheikh Ahmed, the new president of Somalia, ifs also due to attend the summit after his movement won a parliamentary vote a day earlier.
Mugabe has accused the MDC of being orchestrated by Britain and the US, who have repeatedly called for the 84-year-old president to step down.
David Miliband, the British foreign secretary, said the new government would "be judged on its actions, above all by the people of Zimbabwe".
"We are a bit sceptical. These types of things have been announced before," Robert Wood, the US State department spokesman said.
The European Union last week tightened sanctions on Harare by adding more than 60Mugabe allies and companies to an assets freeze that bans them from travelling to, or trading with, EU nations.
The number of companies banned was sharply increased and for the first time included European-based firms.
The EU claimed most of the blacklisted entities were fake companies used to bring money to Mugabe's government and supporters.