The British Foreign Office said the number is likely to rise.
"These appalling terrorist attacks that take place where the perpetrators claim they do it in the name of a religion - they don't," Cameron said.
He called it "an absolutely sickening and despicable attack of appalling brutality".
"They do it in the name of terror, violence and extremism and their warped view of the world. They don't represent Islam or Muslims in Britain or anywhere else in the world."
The Somali militant group al-Shabab says it carried out the attack on the upmarket Westgate shopping centre in response to Kenyan military operations in Somalia.
Cameron said "because the situation is ongoing we should prepare ourselves for further bad news".
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