The top U.S. envoy to Africa said Thursday that Zimbabwe's opposition leader won his nation's disputed presidential election and longtime President Robert Mugabe should step down.

The opposition has claimed its leader, Morgan Tsvangirai, beat Mugabe outright March 29. Independent Zimbabwean observers also say Tsvangirai won, though not by enough to avoid a run-off — and Jendayi Frazer, assistant U.S. secretary of state for African affairs, cited those figures when she spoke to reporters Thursday.

Zimbabweans still await the results. The opposition accuses Mugabe of withholding them while he plots how to keep power and orchestrates a campaign of retribution that the opposition says has killed at least 10 of its supporters. The octogenarian Mugabe has led Zimbabwe since independence in 1980.

Frazer was responding to questions about whether some kind of power-sharing agreement could resolve the impasse.

"We think in this situation we have a clear victor," she said. "Morgan Tsvangirai won, and perhaps outright, at which point you don't need a government of national unity. You have to accept the result." Independent tallies gave Tsvangirai 49.4 percent of votes, a projection that, with a margin of error, means Tsvangirai could have won more than the 50 percent plus one vote needed for outright victory.

Of Mugabe, Frazer said: "He contested for president and he lost. ... President Mugabe should respect the will of the people and allow a new president to come in." That new president should be Tsvangirai, she added.

Frazer was speaking in South Africa at the start of a visit to increase international pressure on the government in Zimbabwe, where human rights leaders and church leaders have reported a brutal campaign against people and communities that voted against Mugabe and his ruling party. Hundreds of people have been tortured and assaulted and hundreds of homes burned, according to their reports.

"The U.S. is increasingly concerned about violence and the human rights abuses taking place in Zimbabwe. This is creating an environment of intimidation and violence," Frazer said.

She was with the U.S. ambassador to Zimbabwe, James McGhee, who said about 1,000 people have been displaced in the violence and that hospitals are unable to cope with the growing numbers of victims.

McGhee said his embassy had proof that granaries have being burned to intimidate people — an outrage in a country where two-thirds of the people are dependent on international food aid.

Zimbabwean civil rights and civic groups presented a dossier, including pictures of victims with broken limbs and missing teeth, to election observers from the Southern African Development Community, according to Fambai Ngirande, spokesman for the National Association of Non-Governmental Organizations. He said one group among the 20 had documented 486 cases of assault and torture, including an opposition supporter stabbed to death.

Ngirande said they asked the economic bloc to intervene to halt "high prospects of civil strife."

Still, Frazer said, there was room for compromise: "There may need to be a political solution, a negotiated solution."

Zimbabwe's state-controlled Herald newspaper, a government mouthpiece, on Wednesday floated the idea of a national unity government led by Mugabe to organize new elections.

But Frazer said "any such government would have to be led by Morgan Tsvangirai as the clear winner of the most votes."

She said the actual results of the elections may never be known. And "now, more than three weeks after the elections ... it is hard for us to accept any results as credible."

Frazer was to meet Tsvangirai later Thursday. South African President Thabo Mbeki, who has been mediating the Zimbabwe crisis with a "quiet diplomacy" criticized in many quarters, is out of the country.

The U.S. envoy is scheduled to travel to Angola to meet with President Eduardo dos Santos, a staunch ally of Mugabe who has not held elections since 1992, and to Zambia for talks with President Levy Mwanawasa. Mwanawasa, considered critical of Mugabe, is current head of the Southern African Development Community of 15 nations, which is thought to have some sway over the intransigent Zimbabwean leader.