The United States on Saturday said all sides must recognize "serious flaws" in the vote-counting process of Kenya's disputed election, and urged ruling and opposition leaders to meet immediately and "without preconditions" to break the cycle of violence.

Jendayi Frazer, assistant secretary of state for African affairs, said that both President Mwai Kibaki and opposition leader Raila Odinga "should acknowledge serious irregularities in the vote tallying which made it impossible to determine with certainty the final result," according to a statement carried online by Kenya's Nation newspaper.

Frazer's statement was issued after a visit to Kenya this week where she worked with other international envoys to resolve an impasse that has led to hundreds dying in rioting and violence across a country that had until now been considered one of the most stable in Africa.

Incumbent Kibaki was declared winner of the December 27 presidential election, but Odinga and electoral observers charged that there were serious problems with the vote tallying.

"The generally peaceful and orderly voting process, and the record voter turnout, was a triumph for the Kenyan people, but the serious flaws in the vote tallying process damaged the credibility of the process," Frazer said.

Mediation efforts by the African Union and others have so far failed. Former UN secretary general Kofi Annan is expected to pick up the mantle of top negotiator when he arrives in Kenya next week.

Frazer warned that the US "cannot conduct business as usual" in Kenya until reconciliation has made significant headway.

She said it was "imperative" for Kibaki and Odinga to "sit together directly and without preconditions to discuss how to end the post-electoral crisis in a way that reflects the will of the Kenyan people."

Odinga has called for a three-day mass rally across Kenya next week in defiance of a government ban on public protests. Human Rights Watch called for the government to lift the protest ban and urged police to show restraint against demonstrators.

Kenyan security forces have a duty to rein in criminal violence and should protect people, but they shouldnt turn their weapons on peaceful protestors," Georgette Gagnon, acting Africa director of the rights group said in a statement. The government should make it very clear that police will be held to account for using lethal force against people for simply expressing political views."

About 600 people have died and 250,000 have been displaced in tribal violence sparked by the disputed election result.