Clinton is due in South Africa later on Thursday, her second stop in an 11-day trip to Africa, and is set to meet President Jacob Zuma in the coastal city of Durban on Saturday. On Friday she is due to meet the foreign minister.
"I do intend to speak not only with President Zuma but other members of his government about what more South Africa believes can be done to strengthen the reform movement inside Zimbabwe, alleviate the suffering of the people of Zimbabwe and try to use its influence to mitigate against the negative effects of the continuing presidency of President Mugabe," said Clinton at a news conference in Nairobi.
Zuma has taken a harder line on Zimbabwe than his predecessor Thabo Mbeki, but the United States would like the new South African president do more to quicken the pace of reform in its neighbour.
The United States, troubled by what it sees as an absence of reform in Zimbabwe, has no plans either to offer major aid or to lift sanctions against Mugabe and some of his supporters.
Before any of that can happen, Washington wants more evidence of political, social and economic reforms, a U.S. official told Reuters before Clinton began her seven-nation trip to Africa.
Mugabe, in power since independence from Britain in 1980, is blamed for plunging Zimbabwe into economic ruin. He argues that hyperinflation and a collapsed infrastructure are caused by sanctions imposed by the United States and others.
Targeted U.S. sanctions include financial and visa restrictions against selected individuals, a ban on transfers of military items and a suspension of non-humanitarian aid.