The United States said there were "serious questions" over Zimbabwe's election results, which were released on Friday more than one month after presidential elections were held, the dpa reported.
Zimbabwe's Election Commission said a run-off election would be needed between long-time President Robert Mugabe and his main challenger Morgan Tsvangirai, whose party has already rejected the results.
US State Department spokesman Tom Casey said the final result had "rather serious credibility problems, given the inexplicably long delays and some of the post-election irregularities that have occurred."
The opposition Movement for Democratic Change accuses Mugabe supporters of rigging the vote and launching a campaign of intimidation and violence since the March 29 elections.
The commission said Tsvangirai won 47.9 per cent of the vote to Mugabe's 43.2 per cent, not enough to avoid a run-off.
Casey said it would be "impossible" for Zimbabwe to hold another election given the harassment of opposition leaders.
The government should "cease the kinds of action it's been taking against the opposition before anyone should even think or be able to talk about any kind of run-off election," Casey said.