WASHINGTON (Reuters) - The United States cast doubt on Thursday over the credibility of Zimbabwe's long-delayed election results and said President Robert Mugabe must "call off his dogs" and stop attacks against political opponents.

Zimbabwean officials started verifying results from the March 29 presidential election on Thursday, a move that State Department spokesman Tom Casey said was way overdue.

"There has been an absolutely unconscionable and inexplicable delay in the process of releasing those, and at this point, whatever those results show, they are probably going to have limited credibility," said Casey.

Casey said it was hard to see how there could be a fair run-off election because of current state-orchestrated violence against the political opposition led by Morgan Tsvangirai.

Senior Zimbabwean government sources said on Thursday Tsvangirai has beaten Mugabe in the election but not by the outright majority needed to avoid a run-off ballot with the veteran leader, who has held power for 28 years.

"President Mugabe must call off his dogs and cease his security services and his supporters' attacks on those who are simply trying to express their views," said Casey.

"The government of Zimbabwe owes its people an opportunity to let their voices be heard and the results speak for themselves," he added.

Tsvangirai's party has accused Mugabe's government of launching a campaign of intimidation ahead of the possible second round and said 20 of its members had been killed by pro-government militias.