The top US envoy in Africa gave warning today that Washington was reviewing all of its aid to Kenya because of Bosnian-style "ethnic cleansing".

Amid increasing international concern about the bloodshed in Kenya, Jendayi Frazer, the US Assistant Secretary of State responsible for Africa policy, also warned that those behind the violence might be adopting killing as their main objective.

Separately, President Kagame of Rwanda, which suffered a genocide in 1994, said that intervention by the military may be the only way to halt the Kenyan bloodshed.

More than 850 people have been killed and a quarter of a million displaced since a disputed election on December 27 that saw President Kibaki returned to power. Raila Odinga, the Kenyan opposition leader, insists that the election was rigged.

It emerged today that Kenyan police have been given shoot-to-kill orders in a bid to stem the violence after Kofi Annan, the former UN Secretary-General, launched crisis talks between the feuding leaders in Nairobi yesterday..

“There are four categories of people who will face tough police action: Those looting property, burning houses, carrying offensive weapons, barricading roads,” a police commander told the AFP. “We have orders to shoot to kill these categories of people if they are caught in the act.”

Yesterday military helicopters fired warning shots to stop ethnic fighting in the lakeside town of Naivasha, the latest flashpoint.

Although the United States previously had said it would not threaten deep cuts to its projected $540 million in aid payments this year, Ms Frazer said that neither Mr Kibaki nor Mr Odinga had done enough to halt the violence.

Speaking to reporters before an African Union summit in Addis Ababa, Ms Frazer described the violence she saw during a visit earlier this month to Kenya’s western Rift Valley, where the fighting has pitted the Kalenjin people against Mr Kibaki’s Kikuyu tribe.

Ms Frazer said she did not consider the violence genocide. “The aim originally was not to kill, it was to cleanse, it was to push them out of the region,” she said, explaining that people were told to leave their homes on the threat of death if they did not flee. It “was clear ethnic cleansing in the Rift Valley”.

Now, after weeks of deadly attacks and retaliation, she said “killing may be the object”.

The New York-based organisation Human Rights Watch has said it had evidence that opposition politicians helped direct and organize some attacks in the Rift Valley - which Mr Odinga has denied. Kenyan human rights groups have said some of the worst violence has been perpetrated by paid militias directed by politicians, and cite a long history of orchestrated political violence in Kenya.

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