Addressing African leaders at a conference in Washington aimed at boosting trade with the continent, Rice referred to the "heartbreaking plight" of the Zimbabwean people due to Mugabe's actions and his disputed re-election last month.

"In the Mugabe regime we see the page of history that Africa must turn. A leader for independence which inherited a nation full of promise, but which has devolved into a tyranny that values nothing but power," she said of Mugabe.

Mugabe won a landslide victory last month in a vote that was ultimately boycotted by opposition leader Morgan Tsvangirai and denounced by Western nations. Tsvangirai's Movement for Democratic Change says pro-Mugabe militia have killed at least 113 of its supporters in a systematic campaign of violence.

The 84-year-old Zimbabwean leader, in power since the end of British rule in 1980, blames the opposition for the bloodshed.

The United States has made clear African neighbors such as South Africa should take the lead in putting pressure on Mugabe, and Rice repeated this appeal in her address to the forum, bringing together sub-Saharan African countries that have a trade arrangement with Washington, including South Africa.

"It is hard to imagine how Africa will ever reach its full potential until all of its leaders are accountable too and respectful of the will of its people," Rice said.

"Southern Africa will face perennial instability until the peaceful aspirations of all Zimbabweans are respected and reflected in their government. This is Africa's challenge and Africa must succeed," she added.

South Africa and other African Union members are pressing Mugabe and Tsvangirai to accept a power-sharing deal. African leaders see a unity government as the way to avert a spread of violence and total economic collapse in Zimbabwe, which has the world's highest inflation rate and chronic food and fuel shortages.

U.S. and British attempts to further isolate Mugabe failed in the U.N. Security Council last Friday when a resolution to impose sanctions against Zimbabwe was torpedoed after Russia and China vetoed the move.

The proposed sanctions would have imposed an arms embargo on Zimbabwe as well as financial and travel restrictions on Mugabe and 13 other officials.

The U.S. State Department said on Monday it was consulting with allies such as Britain to come up with new ideas to put pressure on Mugabe and said nations such as China and Russia were on the "wrong side of history" by vetoing the U.N. action.