Kenya's opposition has filed a complaint with the International Criminal Court, accusing the government of crimes against humanity.
Ethnic gangs have set up checkpoints in the Rift Valley
It says the police used excessive force during last week's protests, in which more than 30 people died.
In the latest violence, four people were hacked to death as at least seven were killed in the Rift Valley.
Former UN chief Kofi Annan warned it would be a disaster if Kenya lost its place as a haven of African stability.
Mr Annan was speaking after arriving in Nairobi to help solve a political crisis that spilled into nationwide violence with the announcement of last month's disputed presidential election results.
A poisoned chalice?
More than 650 people have been killed in protests, while 250,000 more have fled their homes.
Arriving in the Kenyan capital on Tuesday, Mr Annan called for dialogue but denied he had been handed a poisoned chalice with his latest peace mission.
"This is a challenge," he said, flanked by Graca Michel, the wife of former South African President Nelson Mandela, and former Tanzanian President Benjamin Mkapa.
"We are here to listen, learn and work with the concerned parties."
President Yoweri Museveni of Kenya's neighbour Uganda is also in the country and has held talks with President Mwai Kibaki.
During his visit, Mr Annan will meet Mr Kibaki, who says he won December's election fairly, and Orange Democratic Movement (ODM) leader Raila Odinga, who insists the poll was rigged.
"Our message to the parties is this: there can be no solution, no peace and stability... without respect for the rule of law," said Mr Annan.
A spokeswoman for the ICC, based in The Hague, said she could not comment on whether the Orange Democratic Movement (ODM) had filed a complaint over the government's handling of the protests.
Government spokesman Alfred Mutua, however, told the AFP news agency that the authorities were planning to file a counter-suit.
"The government is aware that some leaders of ODM planned and executed a mass genocide that we saw in the Rift Valley of this country."
"They should know that very soon, they, as individuals not as a party, will be languishing in jail," he said.
Some 250,000 people have fled their homes
Members of President Mwai Kibaki's Kikuyu community have often been targeted by opposition supporters in the Rift Valley, as well as in Nairobi slums and the port town of Mombasa.
One group were burnt to death as they sought shelter in a church but ODM leaders have denied responsibility and called for calm.
The ODM accuse police of adopting a shoot-to-kill policy during the protests but the police have said they acted with restraint.
ODM leader Raila Odinga says he will only meet President Mwai Kibaki to discuss the crisis if the talks are part of international mediation efforts.
So far, Mr Kibaki has rejected this condition.
The BBC's Amber Henshaw in Nairobi says few are optimistic about what progress will be made.
Mr Odinga wants fresh polls or a recount, while Mr Kibaki says he was rightfully elected.