The top US envoy to Africa has said the forced removal of people from Kenya's Rift Valley after last month's disputed presidential poll was ethnic cleansing.
Assistant Secretary of State for Africa Jendayi Frazer also denounced the continuing violence which has since forced thousands to flee their homes.
She urged President Mwai Kibaki and opposition leader Raila Odinga to focus on mediation efforts to end the crisis.
Formal negotiations between the two parties were due to begin on Wednesday.
But unconfirmed reports in the Kenyan media say the talks, brokered by former UN chief Kofi Annan, have now been delayed until Thursday because neither side can agree on an agenda.
Up to 900 people have died as violence has spread since the presidential poll, which the opposition claims was rigged.
At least nine people were killed in outbreaks of violence throughout the country on Tuesday, following the killing of Mugabe Ware, an MP from Mr Odinga's Orange Democratic Movement (ODM).
Meanwhile, Kenyan Foreign Minister Moses Wetangula has said Mr Kibaki will attend the African Union summit on Thursday, dismissing suggestions that there were more pressing issues for him to attend to at home.
On Monday, ODM Secretary General Anyang Nyong'o called on member states not to recognise what he called the illegitimate and illegal government of Mr Kibaki.
But Mr Wetangula said there was no injunction against the president and so he was obliged to discharge his function as a head of state.
'Cycle of retaliation'
Speaking in Addis Ababa on the eve of the AU summit, Ms Frazer called on Kenyan political leaders to focus on ending their country's political crisis, and urged them to publicly call for an end to the violence.
She said both sides had spent a lot of unhelpful time adopting hard-line negotiating positions in public and inciting political and ethnic unrest.
And she said those guilty of inciting or carrying out the violence should be held to account by the international community if necessary.
"We're calling for an investigation into the inciting of violence as well as an investigation into who is actually killing people," she said.
Ms Frazer described the forced removal of members of Mr Kibaki's Kikuyu tribe, which she witnessed in the eastern Rift Valley region immediately following the election, as "clear ethnic cleansing".
"The aim originally was not to kill, it was to cleanse, it was to push them out of the region," she said.
"I met with the individuals who were victims of the violence - they all said that they were being pushed out of the area, that organised groups came to them and said: 'You must leave your house by a certain time'."
Ms Frazer went on to denounce the continuing ethnic clashes across the country, which have seen Kikuyus launch reprisal attacks on Luos and Kalenjins, who largely backed Mr Odinga in the election.
"The cycle of retaliation has gone too far and become more dangerous," she added, warning that now "killing may be the object".
The UN's special adviser for the prevention of genocide, Francis Deng, has said he is sending one of his members of staff to observe the situation in Kenya.
"At the moment I would not use the word genocide," he told the BBC's Network Africa programme. "At the moment we are concerned about certain atrocities that could conceivably escalate if they're not stopped."
Earlier, Kenyan Internal Security Minister George Saitoti said the security forces would now "act tough" when dealing with those behind the ethnic and political violence.
"We do not want to have the criminals running around and disrupting the activities of this country and I would like to tell those... who have been used to taking laws into their hands... that they are going to face very, very, very serious consequences," he told NTV television.
Mr Saitoti's comments came as three-person teams of representatives from Mr Kibaki's Party of National Unity and Mr Odinga's ODM were due to begin their deliberations in Nairobi.
The negotiations are to be based on a series of proposals drawn up by Kofi Annan and his team, which includes former Tanzanian President Benjamin Mkapa and Graca Machel, the wife of ex-South African President Nelson Mandela.
KENYA MP NEGOTIATING TEAMS
Party of National Unity:
Justice Minister Martha Karua, Mutula Kilonzo, former Health Minister Prof Samson Ongeri
William Ruto, former Kenyan UK High Commissioner Sally Kosgei, former Vice-President Musalia Mudavadi
The former UN secretary general has given the two sides four weeks to resolve immediate political issues, and up to a year to sort out details.
Launching the formal mediation process on Tuesday, Mr Annan warned that the crisis was having a "profound and negative impact" and urged both sides to take the talks seriously or risk losing international aid.
The BBC's Adam Mynott in Nairobi says parts of the country, particularly the Rift Valley and western Kenya, are cauldrons of hatred as a result of inter-ethnic fighting during the past month.
Even if the talks are successful, some of the wounds caused to Kenyan society may take many years to heal, our correspondent says.
Your comments :
I have witnessed my neighbours, mostly from the Kikuyu, Meru and Kamba tribes run for their dear lives from my Luo tribe. One thing funny is that some of them we have been living happily from childhood. All their houses and possessions have been burnt down and looted. things are also not going well with any Luo trying to hide the Kikuyus coz you are either killed or your houses destroyed. Am 29yrs old and all my life I have never imagined seeing our beloved country ending up like this. Can't the international community do anything instead of us being another Rwanda in the making.
Jared Adongo, Kisumu- Kenya
Here in Meru its business as usual.The place is very peaceful and we are praying for the country.We are also contributing food and clothing to assist the victims of violence. God bless Kenya.
Eliud Muriithi, Meru
What is happening now in Kenya is shocking and terrible beyond imagination. My sister and her daughter have had to evacuate Molo but when they came to stay at Kinoo in Nairobi they received eviction and death threats so my house is their nearest and safest refuge for now. They have lived in Molo most of their life and have no real attachment to Kakmega in Western Province.
Chiboli Shakaba, Nairobi, Kenya
The situation in Mombasa and the coast region as a whole is alarming because tourism has been badly affected,leading to closure of many hotels.The implication is that many people have lost jobs and it'll be difficult for the affected families to live,very difficult!!
Amos Kombo, Mombasa, Kenya
I feel so bad as a youth in a country that has 42 tribes,and that intermarriages are amongst us,i have seen a number of my relatives forced to separation from their beloved ones am just wondering when this is gonna end,how it will heal the wounds of those affected directly and indirectly.The displaced who have invested in the areas where ethnic cleansing is on the rise.
alex mathenge, Nairobi
I am in Nairobi but the reports we are receiving are quite alarming.In some of the residential areas organised groups are threatening members of other communities perceived to be their opponents to vacant or face dire consequences.It is indeed unfortunate that a country that was once perceived the hub of democracy in Africa is in the verge of collapse.Please please whatever the world can do, do it now and don't wait for another genocide to occur here in Kenya.
It is so sad to see what is happening in my beloved country. We used to be one people, one nation but not anymore. People who have always lived together as brother and sisters are now turning against each other. I am so depressed and I do not know when this madness will end. We are all Kenyans and we need each other, lets end this mess for God's sake. One Love, One People and One Nation.