Despite Annan's efforts, Mwai Kibaki, Kenya's president, and Raila Odinga, leader of ODM, the main opposition party, continue to bicker over who won a vote observers said was too badly run for anyone to know.
Annan mediated an agreement between the two on Friday to take steps to resolve the crisis within 15 days.
As part of the agreement, Annan's mediation team unanimously chose Ramaphosa to lead talks in the longer term to address Kenya's ethnic and land issues.
Government officials complained that Ramaphosa, the chief negotiator for South Africa's African National Congress in talks that produced a peaceful end to apartheid in 1994, has business links to Odinga.
Meanwhile, clashes in Kenya's western Rift Valley province between groups of youths claiming allegiance to one side or the other, raged over the weekend.
At least seven people were killed in overnight battles between Kisii and Kalenjin communities, Humphrey Nakitare, district commissioner of Sotik town, said.
Arrows and machetes
The fighting continued on Monday, with hundreds of youths attacking one another in an area where 2,000 people have fled their homes during nine days of clashes, Hassan Noor, the Rift Valley provincial commissioner, said.
The youths were armed with bows and arrows and machetes.
Dozens of houses were also burned, witnesses said.
Nationwide, the violence has displaced more than 300,000 people.