Officials from Kenya's opposition party were behind attacks on members of the president's ethnic group and are planning more, Human Rights watch says.
Thousands of people have fled their homes, especially in the Rift Valley
"We have evidence that ODM politicians and local leaders actively fomented some post-election violence," said the lobby group's acting Africa director.
The opposition has denied previous charges of "ethnic cleansing".
President Mwai Kibaki and opposition leader Raila Odinga will meet later, United Nations officials say.
This would be the first time they have met face-to-face since last month's disputed election.
Former UN chief Kofi Annan will also take part in the talks, aimed at finding a solution to the crisis, which has left more than 650 people dead and driven 250,000 from their homes.
The BBC's Adam Mynott in Nairobi says the meeting at a neutral venue in the capital is a breakthrough, although details of the agenda have not been released.
AFP news agency reports that at least 12 people have died in the latest violence - some in the Rift Valley.
The opposition Orange Democratic Movement (ODM) says it has not yet agreed to an official investigation into their claims of electoral fraud.
ODM spokesman Salim Lone dismissed suggestions from the Ugandan presidency that a deal had been reached that could end the crisis.
He said the idea had not been ruled out but it must come as part of former Mr Annan's mediation efforts.
Human Rights Watch says members of President Kibaki's Kikuyu community were targeted following the announcement of his victory.
Their researchers spoke to member of the rival Kalenjin group, who said they were mobilised by their leaders to attack and loot Kikuyu-owned shops and businesses.
Gangs have set up checkpoints, where rival groups are attacked
Local ODM officials and Kalenjin leaders "arranged frequent meetings following the election to organise, direct and facilitate the violence unleashed by gangs of local youth", HRW said.
One ODM official provided a lorry to ferry youths to attack Kikuyus, it said.
"If the leaders say stop, it will stop immediately," one Kalenjin elder said.
"Opposition leaders are right to challenge Kenya's rigged presidential poll, but they can't use it as an excuse for targeting ethnic groups," said Georgette Gagnon, the group's acting Africa director.
The statement said more attacks were being planned on the Eldoret areas of Langas and Munyaka, where many Kikuyu homes remain intact.
AFP news agency reports the death of at least 12 people overnight in several different incidents.
Eight people were killed overnight in the Rift Valley capital, Nakuru, the police told AFP.
It says two were killed in central Limuru, one man was hacked to death in a Nairobi slum and another Kenyan was killed in the western area of Molo.
Rival armed group have set up roadblocks and members of rival communities are pulled from their vehicles and attacked, especially in the volatile Rift Valley.
Uganda's President Yoweri Museveni left Kenya on Thursday and his spokesman said both sides had agreed to an official investigation to report back within three months.
"President Museveni has left this frame-work and the Kofi Annan team can work with this or ignore it, but he is a happy man that his mission has broken the ice," said Tamale Mirundi.
Raila Odinga (left) wants all mediation to go through Kofi Annan
But the ODM spokesman said having multiple negotiation processes was "not efficient".
He pointed out that any suggestion there should be a re-tallying of votes announced by the Electoral Commission of Kenya was out of the question, as the ODM says there was widespread forging and altering of election results forms.
The ODM leader has previously refused to lodge a legal challenge to the polls and has demanded fresh polls.
On Wednesday night, a shot was fired at Mr Odinga's car in central Nairobi, his spokesman said.
He was not in the car at the time and no-one was hurt.
Earlier, Mr Odinga called off a mass protest planned for Thursday in Nairobi.
He said he was responding to a request by Mr Annan.
The former UN secretary-general is being accompanied by Graca Machel, the wife of former South African President Nelson Mandela, and former Tanzanian President Benjamin Mkapa.
Several earlier attempts to get Mr Odinga and President Kibaki to hold face-to-face talks have failed.
The Catholic Church has urged the two leaders to hold their first direct talks since the crisis began.
"We ask President Mwai Kibaki and Honourable Raila Odinga to open their minds and hearts and immediately enter into dialogue," said a faxed statement signed by Cardinal John Njue and 23 bishops.