"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 said.
"We have orders to shoot to kill these categories of people if they are caught in the act."
Al Jazeera's Yvonne Ndege, reporting from Nairobi, said that police officials had told her that was no explicit shoot-to-kill order, but the law allowed police officers to shoot on sight any people threatening life or property.
Pleas for calm
Talks between Kibaki and Raila Odinga, the opposition leader who accuses him of rigging the election, began in the capital on Tuesday with both men calling for calm.
In the first days after the election, the Kikuyu suffered much of the violence at the hands of Odinga's Luo tribe and other minority groups, but has since carried out numerous revenge attacks.
Frazer said that the US wanted an investigation into the violence, including the killing of civilians by police, and said guilty parties must be held accountable.
"There has been an organised effort to push out people from Rift Valley ... It is clearly ethnic cleansing. I don't consider it genocide," she said.
"The cycle of retaliation has gone too far and has become more dangerous."
Separately, Francis Deng, the UN special adviser on preventing genocide and mass atrocities, has warned that leaders responsible for the post-election violence could be held to account for violations of international law.
Deng specified that he was not saying that anything that had happened so far in Kenya amounted to genocide.
"We're not talking the G-word at this point, but the kind of atrocities we're seeing could easily escalate to dangerous levels," he said.
It would have been the second time since the wave of violence began after the December 27 vote that police have been told to shoot to kill.
Similar orders were given in January when police officers came under attack from gangs.
Meanwhile, Amnesty International (AI), the London-based rights group, has called for the protection of several Kenyan human rights defenders and activists who have received serious death threats.
The group, which includes six men and three women, have received a number of anonymous threats in the form of SMS messages, phone calls and emails.
They are now taking precautions for their safety, such as moving house and not making any public statements.
The threats include accusations that they are "traitors" to their ethnicity.
Soldiers armed with assault rifles and whips patrolled the streets of Naivasha in the Rift Valley on Wednesday, the day after military helicopters firing rubber bullets were used to disperse angry crowds.
"Police will henceforth be very forceful on groups of persons carrying out activities that threaten the lives and property of others," Eric Kraithe, Kenya's police spokesman, said.
About 8,000 displaced people remained in a police compound in the town where they have been sheltering since violence erupted there several days ago.