This article was reported by Celia W. Dugger, David Barboza and Alan Cowell and written by Ms. Dugger.

PRETORIA, South Africa  The top American envoy to Africa declared Thursday that Zimbabwe’s main opposition leader, Morgan Tsvangirai, was the “clear victor” over President Robert Mugabe in the nation’s disputed election and called on other countries  including the United States  to help solve the deepening political and humanitarian crisis there.

The diplomat, Jendayi Frazer, the assistant secretary of state for African affairs, said the election results, based on projections by independent monitors, removed the rationale for any negotiated settlement that left Mr. Mugabe in charge, as was proposed Wednesday in an editorial in The Herald, the state-run newspaper.

“This is a government rejecting the will of the people,” Ms. Frazer said, referring to the Zimbabwe electoral commission’s refusal to announce who won the March 29 presidential election. “If they had voted for Mugabe, the results would already have been announced. Everyone knows what time it is.”

The United States has deferred in recent years to South Africa, the region’s most powerful nation, to mediate between ZANU-PF, Zimbabwe’s governing party, and its political rivals. But at a news briefing on Thursday in Pretoria, South Africa’s capital, Ms. Frazer said the severity of the human rights violations by state-sponsored groups against opposition supporters now required the involvement of more players: the African Union, the United Nations and other nations, including the United States.