WASHINGTON, May 2 (Reuters) - The United States said on Friday it was skeptical a fair run-off election could be held in Zimbabwe and that government-backed repression of the opposition must stop before any new round of voting.

Casting strong doubt on election results released on Friday after more than a month of delay, State Department spokesman Tom Casey said: "This isn't a case of better late than never."

"That final tally, I think, has 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 (MDC) won the country's March 29 presidential vote but Zimbabwe's electoral commission said it faced a second round because opposition leader Morgan Tsvangirai failed to score an absolute majority.

Casey questioned whether it was possible to hold a run-off poll when Tsvangirai and his supporters were harassed and abused by officials from the government of President Robert Mugabe, who has ruled since independence from Britain.

"We certainly as a starting point need to see the government cease the kinds of actions it has been taking against the opposition before being able to think or be able to talk about a run-off election," Casey told reporters.

He said the United States would consult closely with other countries in the region over how to proceed.

Washington has voiced strong displeasure over what it sees as unconvincing attempts by neighbor South Africa to resolve the electoral impasse in Zimbabwe, which has suffered an economic collapse. (Reporting by Sue Pleming, Editing by Frances Kerry)

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