A fourth Briton has been killed in an attack at a Nairobi shopping mall staged by Somali Islamist militants, the Foreign Office in London confirmed on Monday.

"We can confirm that a fourth British national has been identified as one of the victims," a spokeswoman told AFP.

Two other Britons and a dual British-Australian national have already been confirmed among those killed in the brutal assault at the Westgate mall, which has so far claimed 69 lives.

Prime Minister David Cameron was due Monday to chair a meeting of Britain's emergency response committee COBRA over the crisis in the Kenyan capital.