The killing of up to 50 Kikuyus, Kenya's largest ethnic group, as they sought shelter in a church in the city of Eldoret fueled fears of deepening tribal conflict in what has been regarded as one of Africa's most stable democracies.
Kibaki, who won a second term in the disputed elections, is a Kikuyu, while his rival Rail Odinga is from the smaller Luo tribe, which says it has been marginalized.
The Kikuyus had fled to the Assemblies of God Church on Monday night after mobs torched homes. On Tuesday morning, a mob of about 2,000 arrived and started burning the church, a Kikuyu told the Associated Press.
On Tuesday, Kibaki called for a meeting with his political opponents, but Odinga refused, saying he would meet Kibaki only "if he announces that he was not elected."
Odinga accused the government of stoking the chaos, telling the AP that Kibaki's administration "is guilty, directly, of genocide."
The U.N. cited Kenyan police as saying 70,000 people had been displaced in five days of violence. Around 5,400 people also have fled to neighboring Uganda, and several hundred people also have fled to Tanzania, officials there said.
Government spokesman Alfred Mutua downplayed the violence, saying it had only affected about 3 percent of the country's 34 million people. "Kenya is not burning and not at the throes of any division," he told the AP.
The Bush administration expressed dismay over the turmoil.
"It's hard-pressed to comprehend here how this could have gone so wrong in terms of Kenya being on its way to some stability and then having this election turn into such a violent situation," White House press secretary Dana Perino said.
In a joint statement, Secretary of State Condoleezza Rice and Britain's Foreign Secretary David Miliband said there were "independent reports of serious irregularities in the counting process."
Rice and Miliband had welcomed news that the African Union would send its chief, Ghanaian President John Kufuor, to mediate the conflict. AU spokeswoman Habiba Mejri-Cheikh said Kufuor was expected in Kenya on Wednesday, but Kufuor's press office told the Associated Press the leader had canceled the visit. Media reports varied on Kufuor's mediation role.
"This is a time where two of the greatest Kenyan patriots, the president and Raila Odinga, need to step forward and work out a practical way forward in the interest of the Kenyan people," U.S. Ambassador Michael E. Ranneberger told BBC radio.
Miliband said he did not know who won the election but that both sides had a responsibility to resolve the conflict. "I very much hope that both Mr. Odinga and President Kibaki will realize that there is nothing to be gained by either of them pretending that this is cut and dried," he told BBC radio.