WASHINGTON (AFP) - The United States on Saturday said results indicating a landslide victory for Zimbabwe's Robert Mugabe were not "credible" and that the electoral process had been "deeply flawed."

"The United States does not believe that the results announced today represent a credible expression of the will of the Zimbabwean people," Secretary of State John Kerry said in a statement.

Official results from presidential and parliamentary polls gave Mr Mugabe, 89, a thumping victory and another five years in office, extending his 33-year rule.

The 61-34 per cent result was a massive blow to Mr Mugabe's longtime opponent Morgan Tsvangirai, who said his Movement for Democratic Change "totally" rejected the vote and would boycott the new government.

The US statement said the elections had been an opportunity for Zimbabwe to move forward on a democratic path and provide a foundation for growth and prosperity, but it cited "substantial electoral irregularities reported by domestic and regional observers."

"Though the United States was restricted from monitoring these elections, the balance of evidence indicates that today's announcement was the culmination of a deeply flawed process," it said.

"There were irregularities in the provision and composition of the voters roll. The parties had unequal access to state media. The security sector did not safeguard the electoral process on an even-handed basis." Mr Mugabe's ZANU-PF had already claimed victory on Friday before the results were declared.

The election ends an uneasy power-sharing government between Mr Mugabe and Mr Tsvangirai, installed in 2009 after another disputed vote.