Assistant U.S. Secretary of State for African Affairs Jendayi Frazer said that Morgan Tsvangirai won the March 29 vote, and that therefore no power-sharing arrangement with longtime President Robert Mugabe was needed.
"We think in this situation we have a clear victor," she told reporters. "Morgan Tsvangirai won and perhaps outright, at which point you don't need a government of national unity. You have to accept the result."
But she added: "There may need to be a political solution, a negotiated solution."
Frazer was speaking in South Africa at the start of a visit to bolster international pressure on Zimbabwe's government. The U.S. has long been among Mugabe's sharpest critics.
The opposition has claimed that its leader Tsvangirai beat Mugabe outright. Independent Zimbabwean observers also say Tsvangirai won, though not by enough to avoid a runoff.
Frazer said she was basing her conclusions on that independent tally.
Zimbabweans are still awaiting the results, with the opposition accusing Mugabe of withholding them while he plots how to keep power. Mugabe, 82, has led Zimbabwe since independence in 1980.
Nearly four weeks after the unresolved presidential election, a state-run newspaper suggested Wednesday that a national unity government led by longtime leader Robert Mugabe could end Zimbabwe's deepening political and economic crisis.
The head of South Africa's ruling party also said Thursday that any proposals for a national unity government in Zimbabwe were premature.
Jacob Zuma, leader of the African National Congress party and the likely successor to South African President Thabo Mbeki, said the international community's focus should remain on publication of results from the presidential election and the conclusion of recounts in 23 legislative elections.
During a visit to London, Zuma said attempting to predict Zimbabwe's future could hinder efforts to break the election impasse.
"Once you begin to ask that question, you are even preventing a possibility of discussing the issue from the beginning," Zuma said. "You are already influencing what the outcome is likely to be."
He also said reports of state-backed violence in Zimbabwe are of huge concern. "That cannot be allowed, it is wrong and absolutely out of order," Zuma said, adding that the ANC would consider issuing a statement of condemnation.
President Mbeki has been criticized for not condemning Mugabe over delays in releasing the election results. Zuma has taken a tougher line on Mugabe.
When Mbeki steps aside from the presidency next year as required by the Constitution, Zuma is expected to succeed him.