A top U.S. envoy urged Kenya's political rivals Saturday to admit problems with presidential voting and agree to meet without preconditions.

The U.S. assistant secretary of state for African affairs, Jendayi Frazer, said in a statement that both President Mwai Kibaki and opposition leader Raila Odinga should "acknowledge serious irregularities in the vote tallying." The envoy said those irregularities make it impossible to determine with certainty who won the election.

The envoy said Washington is deeply disappointed that Mr. Kibaki and Mr. Odinga have not held face-to-face talks, and warned that, in the meantime, the United States cannot conduct business as usual in Kenya.

More than a quarter million people have been left homeless and over 500 have been killed in violence that has occurred since the disputed election December 27.

Former United Nations Secretary-General Kofi Annan will travel to Kenya next week following the failure to broker an end to the crisis by African Union Chairman and Ghanaian President John Kufuor.

The opposition party says it will hold three days of mass protests beginning Wednesday in defiance of a police ban.

Some information for this report was provided by AFP and Reuters.