The Nairobi mall carnage in which several foreigners were killed by Islamist fighters was a "message to Westerners" who supported Kenya, Somalia's Shebab chief said in an audio message Wednesday.

Reclusive rebel supremo Ahmed Abdi Godane said the four-day bloodbath was a warning to Westerners who "backed Kenya's invasion (of Somalia) that has spilled the blood of the Muslims for the interest of their oil companies".

Godane, who called the killing of at least 67 people including the execution of children a "historic operation", said the assault on the upmarket Westgate mall was also a threat of "more bloodshed" if Kenya did not withdraw its troops from Somalia.

The Al-Qaeda linked group he co-founded has already claimed responsibility for the shocking assault on the sprawling complex, but it is the first time Godane, for whom America has offered a $7 million bounty, has issued a public statement.

"The Kenyan people, you went to a war that is not yours and which is against your interests," he said, in a speech peppered with the traditional verses of Somali martial poetry he is renowned to love.

President Uhuru Kenyatta announced an end to the 80-hour bloodbath late Tuesday, with the "immense" loss of 61 civilians and six members of the security forces. Dozens more are reported missing.

"You have squandered your peace and prosperity and lost your boys," Godane added.

"You elected your politicians, you pay the tax to arm the forces of Uhuru that are massacring the Muslims, you supported the decision to go to war."

In one of the worst attacks in Kenya's history, the militants marched into the four-storey, part Israeli-owned mall at midday Saturday, spraying shoppers with automatic weapons fire and tossing grenades.

"Make your choice: pull your forces out of Muslim lands or face more bloodshed on your soil," Godane added.