The top Africa envoy for the United States told reporters in South Africa Thursday that Zimbabwean opposition leader Morgan Tsvangirai was the "clear" victor in last month's presidential election.

U.S. Assistant Secretary of State for African Affairs Jendayi Frazer is visiting southern Africa to consult with the region's governments on Zimbabwe's election crisis. Almost four weeks after Zimbabweans voted in elections on March 29, the government has failed to release the results of presidential polling.

"The most credible results we have today are a clear victory for Morgan Tsvangirai in the first round and maybe a total victory," Frazer said, according to Agence France-Presse. Frazer added that "there should be a change."

Zimbabwe's electoral law requires that a presidential candidate receive more than 50 percent of votes cast to avoid a run-off. Unofficial results and reports have indicated that Tsvangirai received about 50 percent.

According to the Associated Press, Frazer acknowledged the possibility that negotiations between the ruling party, led by President Robert Mugabe, and the opposition may be necessary. "There may need to be a political solution, a negotiated solution," she said.

Reuters reported that Frazer, citing the long delay in releasing results, told reporters: "We now doubt the credibility of any results that would be released."

She added that she supports an arms embargo on Zimbabwe, as proposed by British Prime Minister Gordon Brown.

Brown and the leader of South Africa's ruling African National Congress, Jacob Zuma, met in London on Thursday. In a joint statement, they called for "an end to any violence and intimidation and stress the importance of respect for the sovereign people of Zimbabwe and the choice they have made at the ballot box."