Kenyan President Uhuru Kenyatta has announcing three days of mourning, in response to the attack on Westgate Mall in Nairobi.

Kenyatta was speaking during a televised national address, in which he said the national mourning period would start on Wednesday.

Kenyatta confirmed the death toll as 61 civilians and six security officers, saying five "terrorists" were killed and 11 were detained.

He said three floors of the shopping centre collapsed during "tail end" of the operation by security forces, and more bodies may be trapped in the rubble.

"Fellow Kenyans, we have been badly hurt and feel great pain and loss, but we have been brave, united and strong. Kenya has stared down great evil and triumphed," he said.