A father and two sons were among six people to die in fresh political violence in Kenya, as international mediators flew in today in a bid to end the weeks long bloodshed.

The six latest victims were killed last night in the volatile Molo district of the Rift Valley where rival tribes have clashed repeatedly.

Their deaths brought to at least 63 the number of people killed since Raila Odinga’s Orange Democratic Movement (ODM) called for nationwide protests last week over President Mwai Kibaki’s re-election.

About 700 people have died since the unrest began, many killed in rioting and looting and at least 80 shot were dead by police. At the same time 250,000 Kenyans have had to flee their homes, as supporters of Mr Odinga, Mr Kibaki's defeated opponent and a member of the Luo tribe, mounted demonstrations and clashed with Mr Kibaki's Kikuyu tribe.

Mr Annan was due to arrive in Kenya tonight along with Graca Machel, the wife of former South African President Nelson Mandela, to push for a solution between the feuding political factions.

They will join Benjamin Mkapa, the former Tanzanian president, who, alongside two other former African heads of state, has been shuttling between Mr Kibaki and Mr Odinga for days to broker a deal.

Mr Odinga has remained defiant, pressing on with his challenge to Mr Kibaki’s re-election and making a triumphant appearance in his western electoral strongholds yesterday.

Meanwhile, Mr Kibaki has dug in, naming a Cabinet packed with his closest allies and vowing to crack down on anyone attempting to defy a rally ban.

A previous mission by John Kufour, the Ghanaian President and head of the African Union, failed to get the two men into the same room.

Expectations are low inside Kenya that Mr Annan will succeed. One columnist in the Daily Nation writes: "Short of getting them both in chokehold and banging their heads together, Mr Annan has very little leverage on either President Kibaki and Mr Odinga or their respective entourages of myopic warmongers and sycophants."

Diplomats hope, however, that Mr Annan's years of experience in negotiating in intractable situations such as Darfur and Israel could help to get a process started in which both men could find a stake. A power-sharing arrangement, with Mr Odinga perhaps given the title of Prime Minister - a role that does not exist in Kenya - is seen as one possibility, followed soon by fresh elections.

Today Mr Kibaki rolled out the red carpet for an international visitor he sees as more sympathetic to his position, as Yoweri Museveni, the Ugandan President, arrived in Nairobi saying that he, too, was going to mediate. Mr Museveni was one of only three African leaders to congratulate Mr Kibaki on his re-election.

The Kenyan Government has reacted angrily to what it sees as criticism from abroad. Last night Adam Wood, the British High Commissioner, was summoned for a dressing down for criticising the way that the crisis is being handled.

Mr Kibaki has also sanctioned full-page adverts in newspapers criticising Western powers, the international media and rights groups for stirring up trouble by questioning the election result.

Michael Ranneberger, the US Ambassador, today hit back, dismissing the adverts as scurrilous propaganda. "There is a compelling evidence of serious irregularities in the vote-tallying process from a wide variety of non-partisan sources," he said.

International observers said that irregularities in the count mean that no one can be certain who won.

With both government and opposition accusing one another of genocide, and unrest growing inside the Kenyan police over alleged "shoot-to-kill" orders at protests, Louise Arbour, the UN commissioner for human rights, said last night that she was gravely concerned by the political violence in Kenya and called for an impartial investigation.

"The killings have to be investigated expeditiously and impartially and anyone found responsible for human rights abuses must be brought to justice," she said in a statement. "There must not be, in any case, impunity for what has occurred in Kenya in the past few weeks."

Meanwhile, Kenyan police fired tear gas to disperse yet another public protest today - only this time it was the President's supporters rather than opposition activists who were gassed.

A hundred of Mr Kibaki supporters took to the streets this morning to demonstrate support for the President, only to fall foul of the increasingly draconian clampdown on street protest. "Kibaki is our president! Lead on, Kibaki," they chanted, before recoiling in panic in a cloud of gas.

Julius Kuria, 32, a market trader in Nairobi's central business district, said that the opposition simply had to accept that Mr Kibaki had been elected fairly.

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