By C. Bryson Hull

NAIROBI, Jan 29 (Reuters) - Former U.N. Secretary General Kofi Annan set Tuesday as a target for Kenya's government and opposition to name negotiators to try to end tribal violence in which more than 800 people have been killed.

Machete-wielding supporters of President Mwai Kibaki and opposition leader Raila Odinga have been fighting each other in Kenya's Rift Valley since late last week in the latest outbreak of violence over a disputed Dec. 27 election.

Nearly 100 people have been killed in the new bloodshed that has been largely centred on the Rift Valley towns of Naivasha and Nakuru, better known for their wildlife-filled lakes. But there were signs the violence was spreading westwards.

Police fired in the air to keep members of Kibaki's Kikuyu tribe, long-dominant in political and business life in East Africa's biggest economy, and Odinga's Luo tribe apart in the tourist spot of Naivasha.

The violence since the election has taken on a momentum of its own, with cycles of killing between tribes who have never reconciled divisions left by British colonial policy and exacerbated by politicians.

"What is alarming about the last few days is that there are evidently hidden hands organising it now. Militias are appearing ... the targeting is very specific," Britain's Africa Minister Mark Malloch Brown said on a visit to Kenya.

An official involved in Annan's mediation mission said the two sides had been asked to study a blueprint for talks on ending the bloodshed in Kenya, a key ally of the West in its efforts to counter al Qaeda.

Annan's mission is part of an African peace effort.

Both sides have traded accusations of genocide in a standoff that has shocked world leaders, who had long viewed Kenya as a peacemaker, rather than a problem, on a volatile continent.

About 250,000 people have been turned into refugees by the violence.

Official results showed Kibaki narrowly won the election but Odinga says victory was stolen from him by vote-rigging. International observers said the poll was flawed.

The United States believed the new violence "underscores the urgent need" for a political agreement, U.S. State Department spokesman Sean McCormack told reporters in Washington. (Additional reporting by Joseph Sudah in Nairobi, David Lewis in Naivasha and Sue Pleming in Washington; editing by Ralph Gowling)

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