(CNN) -- Zimbabwe's opposition leader won the disputed March 29 presidential election, and President Robert Mugabe should step down, the top U.S. envoy to Africa said Thursday.
U.S. envoy Jendayi Frazer said Morgan Tsvangirai was a clear winner in the election.
Assistant U.S. Secretary of State for African Affairs Jendayi Frazer was speaking in South Africa at the start of a visit to increase international pressure on Mugabe.
Frazer joins a growing chorus of international voices criticizing the handling of Zimbabwe's presidential and parliamentary elections.
She said, "My preferred option would be that the will of the people will be accepted. That Mr. Mugabe does the honorable thing and steps down." Watch Frazer's strong statement on Mugabe »
British Prime Minister Gordon Brown; Jacob Zuma, the leader of South Africa's governing party; and Human Rights Watch have all criticized the election and its aftermath.
But Frazer is one of the first U.S. officials to take such an emphatic stance on the issue.
Zimbabwe is locked in a political crisis as the government refuses to release the results of an election that opposition leaders claim as a victory for its candidate Morgan Tsvangirai.
A recount is currently taking place for the presidential election and a partial recount of the parliamentary votes.
The opposition Movement for Democratic Change says Tsvangirai won the election. Mugabe's Zanu-PF party alleges that the opposition is engaged in election tampering.
Frazer said, "We are certain that Morgan Tsvangirai won based on the reporting of the Zimbabwe Electoral Support Network and other independent monitors.
"I think [Mugabe] is -- with this great delay in announcement -- trying to steal the election. He is intimidating population and election officials as well. [We've] also heard reports of the police stuffing ballots."
Frazer also raised the possibility that Mugabe was no longer running Zimbabwe and that it was the security forces which are refusing to acknowledge the election result.
"There are reports that are generals are the ones who said to him, 'We will not accept your defeat; we will not accept victory of Morgan Tsvangirai.' That is what is being reported," she said. "And the way the security apparatus has fanned out across the country, many are saying there has been a quiet coup d'etat.
Meanwhile, a Chinese ship that drew international condemnation for attempting to deliver weapons to Zimbabwe amid the country's political crisis is returning home without offloading its cargo, officials in Beijing said Thursday.
Foreign Ministry spokeswoman Jiang Yu said the vessel was being diverted back to China because Zimbabwe had "failed to receive the cargo as scheduled."
The An Yue Jiang had attempted to make delivery to landlocked Zimbabwe, where violence continues in a dispute over election results, via South Africa but was blocked by a court ruling preventing transport of the arms across the country.
The opposition, which says it is being targeted in attacks by government forces, claims that the weapons would have been used to intimidate voters in the event of a run-off election as Mugabe attempts to shore up his support.
As the ship left South Africa in search of another port, the United States had urged other southern African countries not to allow it to dock. British Prime Minister Gordon Brown also called for an arms embargo on Zimbabwe.
Deputy U.S. State Department spokesman Tom Casey said the United States did not think it was "appropriate for anyone to provide additional weapons in Zimbabwe as they are going through a political crisis."
China defended the arms deal Thursday, saying it was finalized a year ago, before Zimbabwe's current troubles escalated, and called Washington's pressure unjustified.
"There are some people in the United States who would like to pose as the world's policemen, but they are not welcomed in the world," Jiang said. "We have friendly relationship with African countries."
David Cockroft, general secretary of the International Transport Workers Federation, said last week that the shipment includes 3.5 million rifle rounds, small arms, mortar shells and rocket-propelled grenades.
Another U.S. administration official said the ship's cargo was bound for Zimbabwe's security forces.
The ship also tried to dock in Mozambique but was refused permission, the official said.
China is a major small arms supplier for several countries, but the U.S. official said "the timing of this arms shipment is important" given the instability in Zimbabwe. Tensions are high there as a result of the government's refusal to release the results from last month's presidential elections. E-mail to a friend